362 ZOOLOGY 



SECT. 



culata (C), and in Rhynchonella (B) among the Articulata, each 

 arm of the horse-shoe is coiled into a conical spiral, which in some 

 cases can be protruded between the valves. In all cases a section 

 of the arm shows the same general character, with a groove, one 

 of the lips of which is a simple ridge, while the other is the row of 

 tentacles. 



The most noteworthy point about the muscular system is the 

 fact that the shell is both opened and closed by muscular action. 

 The dorsal valve may be taken to represent a lever of which the 

 hinge-line is the fulcrum, the cardinal process the short arm, and 

 the main portion of the valve the long arm. The mode of action 

 of the three principal sets of muscles adductors, divaricators and 

 adjusters has been described in the account of the example (p. 357). 

 In Lingula there is a very complex muscular system by means of 

 which the valves can be rubbed upon one another, or moved laterally 

 as well as opened and shut. 



In the Articulata the enteric canal is V-shaped, as in Magel- 

 lania, the intestine being straight or nearly so, and ending blindly. 

 In the Inarticulata, on the other hand, the intestine is usually 

 coiled, and always ends in an anus (Fig. 301, C, a), which generally 

 opens into the mantle-cavity, but in one genus (Crania) into a 

 pouch or sinus at the posterior end of the body between the 

 valves. 



A heart is usually present, but the function of blood is per- 

 formed mainly by the coelomic fluid, which is propelled by the 

 cilia lining that cavity, and circulates both in the ccelome itself and 

 in the pallial sinuses, each sinus presenting in Lingula at least 

 both an outgoing and an ingoing current. 



A single pair of nephridia, resembling those of Magellania, 

 occurs in all known genera except Rhynchonella, in which there are 

 two pairs, one dorsal and one ventral. Besides discharging an 

 excretory function they act as gonoducts. 



The nervous system always takes the form of a circum-ceso- 

 phageal ring with ganglionic enlargements, the largest of which 

 is ventral or sub-cesophageal in position. Otocysts have been 

 described in Lingula, rudimentary eyes in Megerlia, and patches 

 of sensory epithelium in Cistella : with these exceptions sensory 

 organs are unknown. 



There are usually four gonads, two dorsal and two ventral, 

 sending prolongations into the pallial sinuses. Some genera are 

 dioecious, others hermaphrodite, the epithelium of the gonads 

 producing, in the latter case, both ova and sperms. 



The embryology and larval history of the Brachiopoda 

 have been worked out only in four genera belonging to the Articulata 

 and one (Lingula) belonging to the Inarticulata. The following 

 are the chief general characteristics of the development in the 

 former, In three of the four genera referred to the earlier stages 



