536 



LECTURE XXII. 



195 



surface of the body, as in the Brachiopods. The shell is bivalve in 

 form, but the two valve-shaped plates are confluent at the part 

 representing the hinge («, b, c), and their free borders do not correspond. 



In Jiff. 195, No. 1 shows the 

 ventral surface of the Hi/- 

 alcBa, with the head w, and 

 its wing-like muscular ex- 

 pansions I, m. The hinder 

 or abdominal division of the 

 body is protected by the 

 shell, which terminates be- 

 hind in three points, a, 

 b, c. The ventral plate d 

 is the shortest and most 

 convex : the dorsal plate, 

 2, is nearly flat and oblong. 

 The lateral borders of the 

 two plates, c e, bf^ are sepa- 

 rated by a long and narrow fissure, through which the extensile borders 

 or appendages of the mantle can be protruded. The head and its fins 

 project from the wider anterior opening. The fins /, m, are supported 



on a short and thick neck or 

 pedicle (3, c) : in the living 

 animal they are of a bright yel- 

 low colour with a deep violet 

 spot near their base, which, with 

 their flapping movements, gives 

 them a resemblance to the wings 

 of a butterfly. Between the fins 

 are two small labiate folds, in- 

 cluding the mouth and the outlet 

 of the penis. 



A cylindrical muscle {fig- 196, 

 u) arises from the middle point 

 {a) of the shell, and traverses the 

 visceral mass to be inserted into 

 the neck ; it divides for that 

 purpose into four short fasciculi, 

 which diverge in the substance 

 of the fins : other strata of mus- 

 cular fibres decussate the pre- 

 ceding obliquely in those loco- 

 motive expansions. The nervous 



196 



