536 



LECTURE XXII. 



195 



Hyalaea. 



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 (a, 6, c),and their free borders do not correspond. 



In Jig. 195, No. 1 shows the 

 ventral surface of the Hy- 

 alcea, with the head n, and 

 its wing-like muscular ex- 

 pansions /, 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, hf, 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 Z, w, 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 



