2o6 ZOOLOGY 



them from those which are simply diverticula of the body-wall. 

 These cerata often coexist with well-developed gills, and their 

 minute structure does not point to any very definite respiratory 

 function ; it seems not improbable that their varied shape and 

 colour may be in some cases protective and in others con- 

 spicuous and warning. In those Opisthobranchs which possess 

 hepatocerata, such as Boto and Eolis, the liver is not a compact 

 gland, but consists of a number of diverticula given off from 

 the alimentary canal, each diverticulum passing into one of the 

 cerata, and being large enough for food particles to pass into 

 it and be there digested. In Eolis the liver diverticula do not 

 end blindly, but are stated to open into an invagination of the 

 ectoderm termed the cnidophorous sac. This opening is guarded 

 by a minute sphincter muscle. The cnidophorous sac, which in 

 its turn opens to the exterior, is lined by a number of large cells, 

 cnidoblasts, which are crowded with nematocysts or thread-cells ; 

 these recall the stinging organs of the Coelenterata. The 

 everted threads of these nematocysts are armed with both 

 large and small spines. 



In Aplysia, Bulla, and Fleurobranchus the original cten- 

 idium has been retained, but is situated behind the heart, in Boris 

 and its allies the ctenidium appears in a modified form as a 

 circlet of feathered processes which surround the median dorsal 

 anus ; in Eolis, Tethys, etc., it has completely disappeared. In 

 Aplysia there is a large gland, whose secretion is said to be 

 poisonous, which opens just below the osphradium near the 

 anterior end of the ctenidium ; and numerous small cutaneous 

 glands open on the under surface of the mantle, in Aplysia 

 these produce a purple secretion. 



The Opisthobranchs are, like the Pulmonata, hermaphrodite ; 

 and the generative organs • consist of a hermaphrodite gland or 

 ovo-testis. Some of the cells of this gland form ova, whilst 

 others divide up and become spermatozoa. Prom the ovo- 

 testis a hermaphrodite duct leads to an albuminiparous gland, 

 in the substance of which the duct coils. Just where the 

 duct leaves the gland it gives off a small diverticulum, the 

 vesicula seminalis. The duct then passes on to the external 

 opening, but just before it reaches that it receives the duct of 

 a spherical spermatheca. When eggs leave the body by means 



