XIII 



DECAPODA 



;87 



apical line^ which extends from the extremity of the phragmocone 

 to that of the rostrum. Distribution^ see p. 380. 



Fam. 1. Spirulidae. — Arms with acetabula, shell a loose 

 spiral, without rostrum or pro-ostracum, partially external, 

 enclosed in two lobes of the mantle (Figs. 247 and 248). 



The single species of the single genus (aS'. Peronii Lam. = laevis 

 Gray) has not yet been thoroughly investigated, although the 

 shell occurs in thousands on many tropical beaches, and is some- 

 times drifted on our own shores. The animal appears to have 

 the power of adhesion to the rocks by means of a terminal sucker 

 or pore. The protoconch is present, 

 and contains a prosiphon, which does 

 not connect with the siphuncle. The 

 septal necks are continuous, not broken 

 as in Nautilus. The siphuncle is on 

 the ventral margin of the shell, the 

 last whorl of which projects slightly 

 on the dorsal and ventral sides, but is 

 even there covered by a thin fold of 

 the mantle. The retractor muscles of 

 the funnel and of the head find their 

 point d'appui on the shell, the last 

 chamber of which contains the pos- 

 terior part of the liver, with which the 

 membranous siphuncle is connected. 



Fam. 2. Belemnitidae. — Arms 

 hooked as in Onychoteuthis^ fins large ; 

 phragmocone straight, initial chamber 

 globular, larger than the second, ros- 

 trum often very long, investing the 

 phragmocone, pro-ostracum sword- or 

 leaf-shaped, rounded in front, seldom 

 preserved, ink-sac present. — Lower 

 Lias to Cretaceous. 



The Belemnitidae are believed to have been gregarious, and to 

 have lived in shallow water on a muddy bottom. Specimens are 

 sometimes found in which even the ink-sac can be recognised in 

 situ. The relative proportions of rostrum and phragmocone vary 

 greatly in different groups, the rostrum being in some cases 

 two feet long, in others only just enclosing the phragmocone. 



Fig. 246. — Sepia officinalis L., 

 with mantle cut away to 

 show position of internal 

 shell. X \. (The ends of the 

 tentacular arms are cut off.) 



