7 



IV. ECHIISrOIDEA (Sea Urchins).— (1) Mostly spheroidal; (2) skeleton 

 composed of fixed plates, which form a complete skeleton for the laody, and 

 which are studded over with moveable spines; (2) ambulacral (tube) feet 

 extend from the mouth (ventral) to the apex of the shell, e.g. Echinus 

 (sea urchin). 



V. HOLOTHUEOIDEA (Sea Cucumbers).— (1) Worm-like; elongated; 

 (2) limy skeleton is greatly reduced, and often consists merely of scattered 

 and minute limy plates ; (3) tube-feet may be scattered irregularly over the 

 ventral surface, or may disappear altogether, or may be radially arranged 

 round anterior end; (4) tentacles around the mouth contain prolongations 

 of the water-vascular ring round the gullet, e.g. Holothuria (sea cucumber). 



Class VI.— MOLLUSCA (Shell-fish). 



Characters. — Mostly bilaterally symmetrical invertebrata, covered by 

 a soft skin, a part of which {the mantle) usually secretes a shell, and the 

 ventral surface is drawn out into a muscular outgrowth (the foot). Nervous 

 system mostly in the form of three pairs of chief ganglia (triangular nervous 

 system as opposed to linear type in Worms and in Arfchropoda). 



SXTB-X>IVISIONS. 



I. LAMELLIBRANCHIATA (Leaf-like Gills).— Mollusca, with Uluhed 

 mantle, which secretes a bivalve shell. Head reduced or absent. A series 

 of respiratory filamentous processes grow out on each side of the body 

 (the leaf-like gills). Heart; two auricles; one ventricle. The valves of 

 the shell closed by special (adductor) muscles, e.g. Anodon (fresh-water 

 nmssel), two adductors ; Ostrea (oyster), one adductor. 



II. ODONTOPHORA (Tooth-bearers).— Mollusca, mostly with a more 

 or less distinct head, with sense organs (tentacles and eyes). In the mouth 

 is a rasping organ (tongue or odontophore). Shell, if present, is univalve. 

 The three chief ganglia more aggregated than in Lamellibranchiata. 



1. PoLYPLACOPHOHA. — Primitive and worm-like moUuscs. Breathe by 



gills. Shell consists of eight pieces arranged one behind the other. 

 No cephalic ganglia, e.g. Chiton. 



2. ScAPHOPODA. — No head. The shell is tubular, and open at both 



ends. e.g. Dentalium. 



3. Gasteropoda. — Head, with sense organs and toell-developed foot. 



Heart; one auricle, and one ventricle (usually). Asymmetrical on 

 account of body having undergone a tivisting round its central a.ds. 



(a) Prosobranchiata. — Gasteropoda, with gills in front of heart. 

 Foot well developed, and often with operculum. e.g. Patella 

 (hmpet). 



(6) Opisthobranchiata. — Gills behind the heart. Foot incou' 

 spicuous. Hermaphrodite, e.g. Doris. 



