ORDER PtECILOPODA. 119 



may be regarded as a degenerate or pigmy race." Mr. Thomp- 

 son has traced the gradual developement of these curious ani- 

 mals, which nearly resemble those of the branchipus. 



2. The Aspidiphora, having sixty pairs of legs, all furnished 

 at the base with a vesicle, the anterior very large and ramose, 

 three sessile eyes, with two bivalve capsules inclosing the 

 eggs, and attached to the eleventh pair of legs. The body in 

 the singular typical genus Apus is covered by a large and flat- 

 tened membranous plate of an oval form, with a deep cleft 

 at its hinder extremity, and bearing in front two large eyes, 

 placed close together, with a smaller one behind. 



These animals are occasionally found in considerable numbers 

 in fresh water, particularly such as is stationary ; in which situa- 

 tions they are sometimes observed to be produced in so instanta- 

 neous a manner that it is difficult to conceive whence they could 

 have been brought ; this is more especially the case in puddles of 

 water caused by heavy rains. It is evident, however, that the eggs 

 of these animals must be endowed with great powers of vitality, 

 by which means alone we are able to account for their sudden ap- 

 pearance. They subsist chiefly upon tadpoles. The distinction of 

 the sexes has not been observed, some naturalists even supposing 

 them to be hermaphrodite. When full grown they are about au 

 inch and a half long. 



ORDER VII. PCECILOPODA. 



This order has the legs of variable form, the anterior 

 of an indefinite number, being formed for walking, or 

 prehensile, whilst the posterior are lamelliform or pinnated, 

 and either natatory or serving as organs of respiration 

 (branchite). It is, however, especially by the absence of jaws 

 and under-jaws that they are distinguished from the other 

 Crustacea, these organs being either replaced by the basal 

 part (coxa) of the six pairs of forelegs, which are furnished 

 with minute teeth, or the oral apparatus, consisting in an ex- 

 ternal siphon, in form like an inarticulated rostrum, or in 

 other suctorial instruments, whose structure is not well deter- 



