THE ANIMAL KINGDOM 435 



excurrent tube from the mantle cavity. The mantle is 

 unpaired. A shell may or may not be present. The highly 

 developed paired eyes strongly resemble those of vertebrates. 

 Statocysts and osphradia are also present. Except in Nautilus, 

 there is a glandular ink sac opening near the end of the rectum 

 that secretes a brown or black fluid. The ink is expelled from 

 the mantle cavity through the siphon and affords protection 

 by clouding the water. The sexes are separate. 



Fig. 255. — Mya, the soft-shelled clam. E, excurrent opening of syphon; F, foot; 

 I, incurrent opening of syphon; M, mantle; S, syphon. 



Examples: Loligo pealei, the common squid, its shell (cut- 

 tlebone) being embedded in the tissues; Octopus bairdi, a com- 

 mon devilfish (no shell); Architeuthis princeps, a squid, the 

 largest mollusc, having a body about 6 meters in length and 

 tentacles up to 10 to 12 meters long. Nautilus pompilius, 

 pearly nautilus, which has a shell divided into compartments. 

 It has about 90 tentacles about the mouth. Argonauta argo, 

 the paper nautilus, has a spiral shell without septa. 



PHYLUM 16— ECHINODERMATA 



The term Echinodermata (hedgehog-skinned) was originally 

 applied to sea urchins, for which it was very appropriate, but the 

 term is now used to include in addition to sea urchins all forms 

 related to them, such as starfishes, sea lilies, brittle stars, and sea 

 cucumbers, some of which lack spines and are not in the least 

 "hedgehog-skinned." Echinoderms are marine animals having a 

 radial symmetry, usually of a five-radiate plan. The larva is 

 bilaterally symmetrical, indicating that the radial symmetry of the 

 adult is secondary. Calcareous plates, with or without spines, are 

 developed in the mesoderm, and may form a hard exoskeleton. 

 An ambulacral water-vascular system is present. In the starfish 

 this system begins on the aboral surface with a porous plate, the 



