ANIMAL — REPRODUCTIVE SYSTEM. 
141 
system is develoiied in two chief layers : the Somatic and the 
Splanclinic, which are separated hy the coelomic or hlood space, and 
are therefore practically independent muscnlar tubes, the somatic 
layer forming the external tube 
or body wall, and the splanchnic 
the internal tube or alimentary 
layer. The somatic, which is 
subjacent to, or beneath the 
Fig. 300. — Pseudo-striate muscle fibre from 
buccal bulb of Neritina Jlu7'iatilis, highly 
magnified (after Boll). 
external epithelium and more or less interlaced noth the connective 
tissue, is usualty the most developed, and, in addition to the 
transverse or annular fibres, has more deeply seated longitudinal ones, 
while radiate and oblitiue fibres are also present. The splanchnic layer 
which chiefly surrounds the alimentary canal is more delicate, hut 
exhibits the same arrangement of annular and longitudinal muscular 
bundles as the somatic layer. 
This muscular tissue may become of a firmer and denser texture 
and show fibrous, membranous, or carfilaginous structure, and form 
the powerful muscular bauds by which the animals are attached to 
and withdrawn within their protecting shell. 
Reproduction in mollusca is not accomplished by any of the 
asexual methods which sometimes obtain in other lowly organized 
animals, but is always the result of the complete activity of both the 
male and female organs. The gonads or genital glands are usually 
more or less imbedded within the liver or digestive gland, the sexual 
elements, the spermatozoon and the ovum, being developed from the 
epithelial walls of the constituent ca?ca. 
The generative organs are developed in two chief tyjies — the 
Dioecious or bisexual and the Monoecious or hermaphrodite type — 
the dioecious type having the two sexes in different individuals and 
often exhibiting a noticeable sexual dimorphism in shell and animal, 
while in the monoecious or hermaphrodite type both sexes are 
combined in the same animal by a superposition of the male upon the 
female system. The prevalence of Proterandry or matiu’ation of 
the male element before that of the female tends to prevent the 
possibility of self-fertilization, which, however, has occasionally been 
known to occur. 
The reproductive system in mollusca is typically and primitively 
dioecious, which is a simpler arrangement than the monoecious or 
hermaphrodite condition, which usually exhibits several specialized 
