202 THE SCAPHOPODA 



(Fig. 183, IV), on which the numerous filiform appendages or 

 captacula are inserted. The extremity of each captaculum is 

 swollen and club-shaped with a small lateral pit, and contains a 

 terminal ganglion and a system of ganglion cells whose prolongations 

 are continued into neuro-epithelial elements situated in the lateral 

 pit. The subradular organ is a ciliated ridge on the ventral side 

 of the buccal cavity opposite to the mandible. The epithelium of 

 this ridge contains nerve end-cells, and beneath it are two small 

 nerves derived from the labial commissure, each ending in a small 

 ganglion. The otocysts are situated in the foot (Fig. 182) on the 

 posterior face of the pedal ganglia. They are innervated from the 

 cerebral ganglia and each contains numerous otoconia, 



5. Repi'odudive Organs. The sexes are always separate. The 

 gonad is unpaired and median, and is extremely long, occupying the 

 whole of the postero-dorsal region of the body, below the retractor 

 muscles (Figs. 18 1,/ and 182, go). It is divided into symmetrical 

 transverse lobes, and its anterior extremity is contracted to form 

 a duct, which diverges to the right and opens into the right kidney, 

 as in the Aspidobranchia. 



III. EMBRYOLOGY. 



The ova are laid singly, and undergo irregular segmentation 

 immediately after fertilisation. The ectodermic cells multiply 

 much more rapidly than the large endodermic cell or entomere, 

 which remains for some time unsegmented (Fig. 184, ma). Finally, 

 the entomere segments in its turn, and the endodermic cells derived 

 from it are invaginated to form a gastrula with a large blastopore, 

 situated at the posterior extremity. The embryo elongates and 

 acquires an anterior apical tuft of cilia (Fig. 15,/), behind which at 

 first two and afterwards three, or sometimes four, parallel ciliated rings 

 are formed. These ciliated rings constitute the locomotory velum, 

 and their number diminishes in proportion as the velum, which is 

 more or less reflected backwards over the body, becomes more 

 prominent (Fig. 185, III). 



The blastopore remains open, and gradually travels along the 

 ventral side towards the anterior extremity of the larva. A shallow 

 shell-gland is formed on the dorsal side and extends right and left 

 over the whole dorsal surface, forming two lateral, parallel, and sym- 

 metrical pallial lobes which extend ventrally and finally unite 

 together (Fig. 1 85, II). In this manner the tubular mantle is formed 

 round the body. The shell secreted by the mantle is at first cupuli- 

 form, but subsequently, like the mantle itself, becomes tubuliform as 

 the result of the fusion of its lateral margins (Fig. 119, B). The 

 expanded embryonic shell may still be sei at the initial extremity 

 in some specimens of Siplwnodentalium. 



