150 MOLLUSCA. 



has no radula. The heart is always well developed. Ocelli are 

 present in most. 



The shell is frequently inequivalve and inequilateral, with 

 either one or two adductor muscles for closing its valves. The 

 apex of each valve is the umbo, and between them is the hinge 

 [cardo], with or without teeth, where the two valves are joined 

 to one another. 



Locomotion is very imperfect in the adult state ; many are per- 

 manently fixed, either by their shells or by a peculiar secretion, 

 one form of which is known as a "byssus," or they bury them- 

 selves in the sand, or bore into timber or rocks. They are mostly 

 very prolific ; in the oyster the number of ova [known as the 

 "spat"] varies from 250,000 to 800,000 in the season ; but they 

 take three years to come to maturity. The young of this class 

 are in their earliest stage ciliated and free-swimming. 



While Huxley sees nothing having the value of orders in this 

 class, Carus gives us ten. Schmarda has five orders divided into 

 two sections Endocardines (Rudistae only) and Exocardines 

 (Monomya, including Ostrea, Pecten, &c. ; Heteromya, Mytilus, 

 &c. ; Isomya, Area, Unio, Chama, Venus, &c. ; and Inclusa, 

 GastrochcBna, Pholas, &c.). Some writers adopt two divi- 

 sions, depending on the number of adductor muscles, those bi- 

 valves with one adductor being called " Monomyaria," and those 

 with two " Dimyaria ;" or, again, we have two divisions, based on 

 a character nearer ordinal rank than the other viz. the absence 

 or presence of siphons (tubular prolongations of the mantle). 



There is said to be 14,000 species, recent and fossil. 



With siphons SIPHONIATA. 



Without... .. ASIPHONIATA. 



Order I. ASIPHONIATA. 

 ATRACHIA. 



Headless mollusks without respiratory siphons. Lobes of the 

 mantle free. 



Most of the Asiphoniata are fixed and motionless, the foot 

 being either small or wanting ; in the former case frequently 

 provided with a byssus-secreting gland, situated at the base of 

 the foot, by which it attaches itself to foreign bodies. 



This order includes the oyster (Ostrea edulis), pearl-oyster [of 

 Indian sesis](Meleaffri r >iamarffaritifera), pearl-oyster [of European 

 rivers] (Margaritana margaritifera), mussel (Mytilus edulis), and 



