246 Mr. Baird on British Entomostraca. 



other, forming the head ; the other, much larger, being the 

 body properly so called, or abdomen. This soft body is con- 

 tained within a very slender and delicate shell, the part cover- 

 ing the head being much harder than the other parts and pro- 

 longed underneath into a considerable sized beak. The valves, 

 which inclose between them the abdomen, are in most of the 

 species perfectly smooth round their circumference, but on the 

 middle are marked with deep crossed lines, forming a mesh 

 work, or as SchaefFer describes it, are shagreened like the skin 

 of the shark. They are open on the anterior margin, and along 

 the posterior extremity as far as the tail, but have no hinge, 

 being, asGoeze says, simply soldered together, though SchaefFer 

 asserts that the animal can open and shut them at pleasure. 

 In some species these valves are prolonged posteriorly to a 

 point forming the tail, which at some periods of their growth, 

 and in some varieties, is very long, in others very short, and 

 in some altogether wanting. In the head we distinguish the 

 following parts : beak, antennae, eye, rami, brain, mouth, and 

 part of the digestive canal. In the body we distinguish part 

 of the digestive canal, the body of the animal itself, heart, legs, 

 and organs of generation. The beak is a prolongation of the 

 hard covering of the head, and is asserted by Swammerdam 

 to be the mouth of the animal, by means of which, being 

 pointed, it sucks up its food. Both DeGeer and SchaefFer how- 

 ever pointed out the erroneous nature of this assertion, and 

 later writers, such as Jurine and Straus, have still more clearly 

 shown it to be wrong. At the extremity of this beak, and a 

 little underneath it, we see two small projecting organs, which 

 are the antennae. SchaefFer, who is perhaps the first person 

 that has noticed these, considered them as palpi, by means of 

 which the insect distinguished food proper for itself. Jurine 

 calls them " barbillons," but Straus considers them as being 

 the true antennae of the insect, though he says they do not 

 seem to possess any voluntary motion. In the female they are 

 extremely small, and from being much larger in the male, 

 Muller, who does not seem to have observed them in the female 

 at all, considered them as the male organs of generation. 

 Jurine describes them very particularly in the male ; he calls 

 them "harpons" (Plate ix. fig. 11), says they occupy the 



