3 1 6 Introduction to Animal Morphology. 



peripharyngeal crescent or ring, analogous, but not 

 homologous to a cranium. In Nautilus it is horse- 

 shoe-shaped, and its cornua end above and below in 

 pointed processes. In Dibranchs it is median, peri- 

 cesophageal,* with two lateral processes, either flat 

 expansions forming roofs for the orbits, or else forming 

 perfect orbital cavities. In Sepia there is also a 

 pointed median cartilage outside the basal part of the 

 arms, and also a semilunar piece in the anterior dorsal 

 region of the mantle. In Octopus the latter is repre- 

 sented by two lateral pieces, with no medial part, and 

 in Loligo it is absent. There is often a neck cartilage, 

 largest in Loligo, absent in Sepiola ; and at the base 

 of the funnel are two hinge cartilages (absent in 

 Octopoda). Along the sides of the lateral fins are 

 two cartilages, long and sword-like in Sepia. The 

 cartilage cells in Sepia send numerous long fine pro- 

 cesses through the intercellular substance, giving it a 

 finely striped appearance. 



On account of the antero-posterior shortening of 

 the body, the foot approximates to, and surrounds the 

 mouth, and has its margin divided either into a cluster 

 of many tentacles (Nautilus), or into 8-10 long sucker- 

 bearing arms. Two epipodial lamellae spring from 

 the posterior side near the head, and, in most, unite to 

 form a tube [funnel) directed downwards and back- 

 wards, with its upper end towards the mantle cavity. 

 The mantle margin overlies it so closely in life that 

 this funnel is the only communication of the mantle 

 cavity with the outer world. The tentacles in Nautilus 

 are triangular in section, and arranged in two circlets, 



* Forming a hollow ring with the nervous system in its transverse 



•cavity. 



