OF SOUTHERN INDIA. 395 
present. The penis and the vagina are placed together in a cavity on the right 
side of the neck. The ova and the spermatozoa are, however, formed in the same 
follicles, the former peripherically, the latter more centrally and basal. The 
connection of the follicles with the sexual organs has not yet been distinctly traced 
out. A reciprocal impregnation seems to be the general rule among OpisrHo- , 
BRANCHIA, but Nordman states that he obtained ripe ova from Tergipes, which 
had been kept solitary, so it is probable that under certain circumstances they 
are also self-impregnating. 
According to observations made on the animal of Zergipes, the embryo after 
leaving the egg is provided with a ciliated fringe and two cephalic fins, secreting 
very early a transparent homogeneous shell, composed of one whorl; in place of the 
foot there is a kind of veil or rudder ; and a very minute operculum is also present. 
The larva thus furnished swims with great activity, but after a short time it drops 
the shell and the operculum, absorbs the fins, the body becomes somewhat 
elongated, and the young animal then resembles in every way the old one. ‘This 
kind of development appears to apply specially to the naked families, but in those 
provided with a shell, like the Acrz#onzpx, the smooth embryonal whorls remain 
preserved and grow by distinguishable strize of growth into a more or less large 
and spiral shell. 
Gray calls this order HeTEROBRANCHIATA on account of the various forms of 
the gills, but he excludes the Acrzonzpz from it, placing them in the PRoso- 
BRANCHIA. 
As to the sub-division of the OpistHoprancHIA most conchologists agree in 
distinguishing two groups, the so called Tecri- and the NuDI-BRANCHIATA. Bronn 
(Klassen und Ordnungen des Thierreiches, vol. III, p. 794) says that this order 
offers a beautiful example of the development of separate organs designed for special 
kinds of work, their relative concentration progressing in the same degree as does 
the whole organism. This progressive development is especially apparent in the 
gills, which, therefore, are very useful in distinguishing sub-divisions. Lowest in 
the series of organisation stands Rhodope, the body having a discoidal shape, with- 
out special gills, heart, or any vascular system. The gradual development of the 
gills may shortly be expressed thus; at first the entire surface of the body acts as a 
respiratory organ, especially at the back, then different appendages are formed for 
the purpose of increasing the space to come in contact with the oxygen; these 
appendages are afterwards differently ramified and form themselves into regular gills, 
being connected with various internal canals and blood-vessels ; these gills which are 
at first spread over the whole body arrange themselves afterwards round the anal 
opening, then again they seek protection under a lappet of the mantle, which at 
last secretes a more or less thickened, flat or spirally coiled shell, the gilts being 
then lodged in a sort of an open cavity. The shell gradually grows spirally to a size, 
sufficient to give full protection to the retracted animal and is closed by an oper- 
culum ; in this state it can hardly be distinguished from the shells of the Proso- 
BRANCHIA, except by its usually punctated surface. 
