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. Reproductive 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. 181, f 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. 
Ill. 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. 185, 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 seen at the initial extremity 
in some specimens of Siphonodentalium. 
