It is, however, fair to state that his system was not built entirely upon these muscular variations, but rather upon a more laborious combination of anatomical characters, which were so selected that they presumably could not stand in direct correlation with each other, notably the oil-gland, caeca, carotids, nasal bones and above all, the muscles of the thigh.
Alimentary canal rarely coiled, occasionally with glands which are simple caeca and sometimes serve as air reservoirs; jaws often present and an eversible pharynx.
The oesophagus is provided often with caeca which in Syllids and Hesionidae have been found to contain air, and possibly therefore perform the function of the fish's air-bladder.
Among the purely aquatic families such structures are very rare, and are represented by two caeca in the genus Limnodriloides.
The intestine has a pair of caeca or two or three pairs (but all lie in one segment) in the genus Pheretima and in one species of Rhinodrilus.
In Benhamia caecifera and at least one other earthworm there are numerous caeca, one pair to each segment.
He was quite aware of the taxonomic value of the vocal organs of some groups of birds, presently to be especially mentioned, and he had himself ascertained the presence and absence of caeca in a not inconsiderable number of groups, drawing thence very justifiable inferences.
In addition to the musculature of the proboscis and proboscidian sheath, longitudinal muscular fibres are found in the walls of the oesophagus, whilst transverse ones are numerous and united into vertical dissepiments between the successive intestinal caeca, thus bringing about a very regular internal metamerization.
Cases of asymmetry or irregularity in the arrangement of the intestinal caeca, though sometimes occurring, are not normal.
Caeca are always eminently regular.
The inner surface of the intestinal caeca is ciliated, the caeca themselves are some times - especially in the UT hindermost portion of the body - of a considerably smaller lumen than the intermediate genital spaces; sometimes, however, the reverse is the case, and in both cases it is the smaller lumen that appears enclosed between and suspended by the transverse fibres constituting the muscular dissepiments above mentioned.
They are metamerically placed, and belong to the same metamere as the digestive caeca, thus alternating with the generative sacs.
On its inner surface the longitudinal canal is adpressed to the lateral bloodvessel, and gives off a number of small, blind caeca or tags, each of which ends in a small clump of cells.
Caeca of the intestine.
It is very probable that in Scorpio they do not serve merely to secrete a digestive fluid (shown in other Arthropoda to resemble the pancreatic fluid), but that they also become distended by the juices of the prey sucked in by the scorpion - as certainly must occur in the case of the simple unbranched gastric caeca of the spiders.
D, Mesosomatic gastric caeca (so-called liver).
Limulus agrees with the majority of the Crustacea in being destitute of renal excretory caeca or tubes opening into the hinder part of the gut.
Further, it is pointed out by Korschelt and Heider that the hinder portion of the gut frequently acts in Arthropoda as an organ of nitrogenous excretion in the absence of any special excretory tubules, and that the production of such caeca from its surface in separate lines of descent does not involve any elaborate or unlikely process of growth.
Sal, Prosomatic pair of gastric caeca in Scorpio, called salivary glands by some writers.
C 1, and c 2, The anterior two pairs of gastric caeca and ducts of the mesosomatic region.
C 8, c 4 and c 1, Caeca and ducts of Scorpio not represented in Limulus.
From all parts of the pyriform sac narrow stalk-like tubes are given off, ending in abundant widely-spread branching glandular caeca, which form the essential renal secreting apparatus.
Fine caeca of the nephridium, which are seen ramifying transversely over the whole inner surface of the pedal muscular mass.
Most commonly there is a pair of lateral caeca, which may be more or less ramified and may form a massive " hepato-pancreas " or " liver."
The absence of such renal caeca in Limulus and their presence in the terrestrial Arachnida is precisely on a parallel with their absence in aquatic Crustacea and their presence in the feebly branchiate Amphipoda.
Renal excretory caeca (Malpighian tubes) are developed from the proctodaeum (not from mesenteron as in scorpion and Amphipoda).
These caeca occur in birds (as in mammals) at the junction of the small with the large intestine; and while in ordinary perching-birds they are reduced to small nipplelike buds of no functional importance, in many other birds - owls for instance - they form quite long receptacles.
The existence of paired caeca was previously known in a few armadillos and anteaters, but Dr Mitchell has shown that they are common in these groups, while he has also recorded their occurrence in the hyrax and the manati.
With the aid of these instances of paired caeca, coupled with the frequent existence of a rudiment of its missing fellow when only one is functional, the author has been enabled to demonstrate conclusively that these double organs in birds correspond in relations with their normally single representative in mammals.
The absence of an apical system of plates; the fact that radial symmetry has not affected the generative organs, as it has in all other recent classes; the welldeveloped muscles of the body-wall, supposed to be directly inherited from some worm-like ancestor; the presence on the inner walls of the body in the family Synaptidae of ciliated funnels, which have been rashly compared to the excretory organs (nephridia) of many worms; the outgrowth from the rectum in other genera of caeca (Cuvierian organs and respiratory trees), which recall the anal glands of the Gephyrean worms; the absence of podia (tube-feet) in many genera, and even of the radial water-vessels in Synaptidae; the absence of that peculiar structure known in other echinoderms by the names "axial organ," "ovoid gland," &c; the simpler form of the larva - all these features have, for good reason or bad, been regarded as primitive.
The comparative anatomy of living forms, combined with the evolutionary hypothesis sketched above, suggests that the early holothurians possessed the following characters: subvective grooves entirely closed; 5 radial canals, proceeding from the water-ring, gave off branches furnished with ampullae to the podia on each side of them, the 10 anterior podia being changed into cylindrical tentacles; the transverse muscles of the body-wall formed a circular layer, probably interrupted at the radii (though Ludwig believes the contrary); longitudinal muscles as paired radial bands, without those special retractors for withdrawing the anterior part of the body which occur in many recent forms; a hydropore connected with the water-ring by a canal in the dorsal mesentery; a gonopore behind the hydropore connected by a single duct with a bunch of genital pouches on each side of the mesentery; gut dextrally coiled, with a simple blood-vascular system, and with an enlargement at the anus for respiration, this eventually producing branched caeca called "respiratory trees"; skeleton reduced to a ring of 5 radial and 5 interradial plates round the gullet, and small plates, with a hexagonally meshed network, dispersed through the integument.
Abruptly from the disk and contain neither genital glands nor digestive caeca; no anus; respiration may be through clefts at the bases of the rays, but not by papulae; skeletal appendages confined to spines, usually of simple structure.