338 HANDBOOK OF ESTVERTEBRATE ZOOLOGY. 



a. On the dorsal surface of the neck notice the dorsal 

 mantle cartilage (Figs. 176, h, 190, I), an elongated, flat- 

 tened, cartilaginous plate, with a groove along the middle 

 of its surface, and a ridge on each side of the groove. 



h. Lying upon this plate, but covered by the integu- 

 ment of the mantle, notice the upper end of the pen 

 (Fig. 190, n), with a longitudinal ridge which fits into the 

 groove in the plate. 



c. On each side of the body the edge of the mantle is 

 produced forwards, forming a tooth-shaped point. 



d. On the inner surface of the mantle, in the same 

 region, is a longitudinal ridge (Figs. 177, e, 190, ^), 

 about an inch long. 



e. On the outer edges of the base of the siphon are 

 two siphonal cartilages (Figs. 176, g^ 111, d, 190, h), 

 each of which carries a longitudinal groove, into which 

 the ridsre on the inner face of the mantle fits. 



f. The head is attached to the mantle by a neck, which 

 is mainl}^ composed of four large muscles, the two dorsal 

 retractors of the head (Fig. 176, I), and the two ventral 

 retractors of the siphon (Fig. 176, Tc). 



g. On each side of the first pair of muscles, just poste- 

 rior to the dorsal mantle cartilage, notice a pair of nerves 

 which pass out from the neck into the mantle, and end in 

 the large ganglia stellata, which supply the mantle with 

 nerves. 



h. The siphon is now seen to be somewhat pyramidal 

 in shape, and wrapped around the neck, with the small 

 ■end pointing forwards ; its cavity is divided into three 

 •chambers. 



1. The funnel-shaped ventral chamber (Fig. 176, e), 

 ■communicating with the mantle cavity at its broad end, 

 and with the small valvular external aperture at the small 

 -end. 



