692 THE ANIMAL KINGDOM. 



larva which does not affect the mesoderm, but owing to the presence of 

 two pairs of nephridia in the genus Rhynchonella. It is not a necessary 

 inference from the fact, inasmuch as there may be more than one pair of 

 nephridia to a somite in some Chaetopoda ; but the difference in position of 

 the two pairs of organs rather lends weight to the view. The presence of 

 a pair of otocysts in the larvae of the hingeless genera, of setae in the adult, 

 but more especially of provisional setae in most of the larvae, are points 

 which should also be noted in the anatomy of Brachiopoda. Setae are not 

 necessarily characteristic of the Vermian Chaetopoda ; e. g. they occur in 

 Chiton among Mollusca, but provisional larval setae are not met with 

 elsewhere. The presence of the structures named tends rather to the sup- 

 position, when coupled with other facts, that Brachiopoda are a group of 

 simplified or degenerated organisms. 



It is better under these circumstances of doubt to retain the four 

 classes in question apart by themselves, but it must be carefully borne in 

 mind at the same time that there is no evidence worth much to connect 

 them one with another. 



CLASS BRACHIOPODA. 



Coelomate Metazoa fixed to some foreign object usually by a pedtmcle, and 

 provided with a bivalved calcareous shell. This shell is inequivalve, but each 

 valve is equilateral (see p. 1 24). Its valves are either free from one another ; 

 or one, the dorsal, is hinged on the other, the ventral. They are lined by two 

 mantle folds or extensions of the body-walls, the free margins of which are 

 generally beset with chitinoid setae. There is a lophophore which typically 

 surroimds the mouth and bears ciliated tentacles or cirri, but is in some cases 

 produced into spirally coiled ' arms' The digestive tract is ciliated and 

 furnished with glands, the liver so-called. The anus is either absent, or 

 when present, is either posterior or lateral. There are one or two pairs of 

 nephridia. The sexes are either separate or united (?). There is a meta- 

 morphosis. Exclusively marine. 



A difference of size between the two valves of the shell is well-marked 

 in all Brachiopoda with the exception of the Lingulidae where it is but 

 slight. The larger valve is termed ventral, or superior ; the smaller, dorsal 

 or inferior 1 . The outline of the valves varies in shape and is rounded, 

 oval, or triangular, but the shell is usually narrowed towards the attach- 

 ment of the peduncle or the line of the hinge, which may be pointed, 

 curved, or straight. The shell may be convex, flattened, or concave, 



1 The terms ' dorsal ' and ' ventral ' are applied by Von Buch and Quenstedt to the larger and 

 smaller valves respectively, i. e. conversely to what is usual in English Manuals. The larger valve 

 appears to have been called ventral because the chief nerve ganglion corresponds to it. In the living 

 animal it lies uppermost except where, as in Crania, it is fixed to some foreign object. 



