3 i4 CHARACTERS OF INVERTEBRATE ANIMALS 



careous material. This is to be regarded as a shell, and is not 

 internal in the same sense as the internal skeleton of a Verte- 

 brate, for it is inclosed in a pouch of the skin which has lost the. 

 opening to the exterior probably possessed by ancestral forms. 

 Some Molluscs still exist in which the shell is almost but not 

 quite covered by folds of the skin which have grown over it. 



As regards the internal structure of the Cuttle- Fish, it need 

 only be remarked that there is a very large rasping organ- 

 (odontophore) ; a systemic heart, consisting of a ventricle and 

 two auricles; and a nerve-ring, swollen into very large ganglia 

 and protected by a cartilaginous case. 



The eggs are enclosed in oval cases which are united together 

 into masses, that have been compared to bunches of grapes in 

 appearance, and which are among the common objects cast up 

 on the sea-shore. 



Cephalopods are divided into two sub-classes, named, accord- 

 ing to the number of gills: i. Dibranchiata (Gk. dis, twice; 

 branchia, gills), of which the Cuttle- Fish is an example; and 

 2. Tetrabranchiata (Gk. tetra, four; branchia), of which the only 

 living representative is the Pearly Nautilus. 



Sub-class i. CUTTLE-FISHES (Dibranchiata) 



This sub-class is again divided into two groups, Decapoda with 

 ten arms, and Octopoda with eight. The former includes the 

 Cuttle-Fishes, of which Sepia is a type, the Squids,, and Spirula. 

 Squids, or, as they are sometimes termed, Calamaries, have an 

 even wider distribution than Cuttle-Fishes, for they are not 

 only found in coastal waters, but are also pelagic, i.e. living in 

 the open sea, where shoals of them are met with. A common 

 Atlantic and Mediterranean species which abounds on our shores 

 is the Common Squid (Loligo vulgaris). This animal is of more 

 slender build than the Cuttle- Fish, and a large triangular fin 

 projects from each side. The shell or "pen " is a narrow horny 

 structure, shaped like a lance-head. The old name Calamary 

 for creatures of the kind was given in allusion to this "pen 5 * 

 (L. calamus, a quill), its shape being compared to a short quill. 



Some of the Squid family attain gigantic dimensions, 

 and there can be little doubt that specimens of the kind are 

 largely responsible for the numerous tales and legends which 



