448 PHYSIOLOGY OF THE INVERTEBRATA. 



ova pass into the body or perivisceral cavity, and from thence 

 they pass to the pallial chamber by the excretory ducts. 

 After fecundation the ovum becomes completely segmented, 

 and a free swimming gastrula is formed by invagination. 

 After this a free swimming, segmented, ciliated larva, like 

 that of the Annelida, is produced. This larva is composed 

 of three segments — the cephalic, thoracic, and caudal; it 

 ultimately becomes fixed, and the shell develops from the 

 thoracic segment. 



The Mollusca. 



In the Lamellihranchiata, the sexes are usually distinct, but 

 these animals are sometimes hermaphrodites.* The genital 

 organs in both sexes are somewhat similar to each other. 

 They are paired, lobed, or racemose glands ; and occupy the 

 upper portion of the foot. Taking Anodonta as a typical 

 example of the Zamellibranchiata, these glands vary in size 

 with the season. In the spring and winter, they are larger 

 than at any other season. The genital ducts (of both sexes) 

 open into the cloaca. The testes are white or yellow (due to 

 the spermatic iluid), and the ovaries are red (due to the colour 

 of the ova). The fecundation of the ovum takes place in the 

 branchial chamber of the female ; it then passes into the 

 branchial spaces of the external gills. The segmentation is 

 unequal, and the embryo passes through a morula (blasto- 

 rneres), a gastrula, and a free-swimming, ciliated, or veligerous 

 condition. 



The Gasteropoda are either dioecious or hermaphrodite.! 

 As a typical example of the Pulmogasteropeda, we describe the 

 reproductive organs of Helix. This animal is hermaphrodite ; 

 and its genital apparatus consists of a single gland — the 

 ovotestis (Pig. 78) composed of branched tubules. In this 



* Cyclas, Oardium (some species), Pecten (some species), Ostrwa, Pandora, 

 &c. 



t The -Scaphopoda and Heteropoda are dioecious, while the Polyplacopliora 

 are hermaphrodites. 



