Appendages of body re „ duced to branchiae, present only in four species, and - - to the ventral copulatory 32 appendages of Alma and Criodrilus.
The ganglia are crowded at the posterior end of the body as in leeches, and there is much tendency to the obliteration of the coelom as in that group. Pterodrilus and Cirrodrilus bear a few, or circles of, external processes which may be branchiae; Bdellodrilus and Astacobdella have none.
The appendages of the body are reduced to branchiae, present in certain forms. A clitellum is present.
F, Gill lamellae (not ctenidia, but organs of the pallial complex, having two kidneys, in some cases two branchiae, and two auricles.
Acmaea, without pallial branchiae, British.
Scurria, with pallial branchiae in a circle beneath the mantle.
No ctenidia but pallial branchiae in a circle between mantle and foot.
Patella, pallial branchiae forming a complete circle, no epipodial tentacles, British.
Helcion, circlet of branchiae interrupted anteriorly, British.
Veil; branchiae non-retractile.
Ill-developed; branchiae generally retractile.
Pharynx suctorial; branchiae surrounding the body, between the mantle and foot.
He has observed that in young specimens of Siren lacertina (the larva is still unknown) the gills are rudimentary and functionless, and that it is only in large adult specimens that they are fully developed in structure and function; he therefore concludes that the sirens are the descendants of a terrestrial type of batrachians, which passed through a metamorphosis like the other members of their class, but that more recently they have adopted a permanently aquatic life, and have resumed their branchiae by reversion.
Mantle open; foot rather small; branchiae folded; shell inequivalve.
The Dibranchia, with only one pair of branchiae, one pair of renal organs, and one pair of genital ducts, are much more recent, not appearing till the end of the Secondary epoch, and therefore must be regarded as descended from the Tetrabranchia.
The largest pair of branchiae is placed immediately behind the renal openings and corresponds to the single pair of other molluscs, the organs being repeated anteriorly only (Metamacrobranchs) or anteriorly and posteriorly (Mesomacrobranchs).
The single pair of branchiae are placed symmetrically right and left of the anus, and each has the structure of a ctenidium bearing a row of lamellae on each side as in the Polyplacophora.
In the Euphausiidae the digitiform-arborescent branchiae, as if conscious of their own extreme elegance, remain wholly uncovered.
In theMysidae the branchiae are wanting, and some would form this family into a separate order, Mysidacea.
The second maxillipeds are developed into powerful prehensile organs, and the branchiae, instead of being connected with the appendages of head and trunk, are developed on the pleopods, appendages of the abdomen.
Edwards (1879), which is unique in having supplementary ramified branchiae developed at the bases of the pleopods.
Gills or branchiae may be developed by parts of an appendage becoming thin-walled and vascular and either expanded into a thin lamella or ramified.
Some of the special modifications of branchiae are referred to below.
When present, the branchiae are generally differentiations of parts of the appendages, most often the epipodites, as in the Phyllopoda.
The podobranchiae are clearly epipodites, or, more correctly, parts of the epipodites, and it is probable that the arthroand pleuro-branchiae are also epipodial in origin and have migrated from the proximal segment of the limbs on to the adjacent body-wall.
The genital papilla of the female acquires a great development during the breeding season and becomes produced into a tube nearly as long as the fish itself; this acts as an ovipositor by means of which the comparatively few and large eggs (3 millimetres in diameter) are introduced through the gaping valves between the branchiae of pond mussels (Unio and Anodonta), where, after being inseminated, they undergo their development, the fry leaving their host about a month later.