CKUSTACEA. 333 



yet succeeded in detecting unequivocal vestiges of legs, are in favour 

 of the supposition that they must have corresponded, in the perisli- 

 able structure of their feet, with the Apus and Branchipus. 



But, to what end, it may be asked, tends all this discussion con- 

 cerning the affinities of animals that have long since ceased to exist ? 

 How are we concerned w^ith it, in considerations relative to the 

 generation and development of the actual Crustacea ? To this I 

 have to answer, that it is only by a knowledge of the transitional 

 larval forms of these, that we come rightly to comprehend the nature 

 and affinities of the extinct Trilobites ; and that our knowledge of 

 the most interesting relations of actual InrvjB requires a previous 

 knowledge of the forms of their class that have heretofore existed 

 in this planet. In no mature stage of any existing Crustacean do we 

 find the trilobites so closely resembled, as they are by the larval 

 stages of the Limulus, and of some of the smaller Entomostraca. 

 The metamorphoses of these are such that different stages are 

 figured as distinct species in the well-known work " On Ento- 

 mostraca," by 0. r. Mliller.* In the larval Cyclops^ wliich is his 

 genus Naiiplius, the body is rounded and without the tail; it has 

 only two pairs of limbs : the eyes are distinct ; but, in the course of 

 the first and second moult they approximate, and finally unite to- 

 gether. The body of the larval Branchipus consists at first of two 

 oval divisions, like the spiders : that which represents the thorax 

 and abdomen united has no legs ; but two pairs of natatory seti- 

 gerous legs, and two antennae, are developed from the head, which 

 has a single median eye. The form of the head is also different 

 from that of the parent. After the first moult the head has three 

 eyes, a pair being added to the first median one ; but all are ses- 

 sile. The abdomen elongates, and divides into rings, and is now 

 provided with rudimentary tubular feet. At the secondary moult, 

 the first pair of foliaceous feet appear, and are succeeded by seven 

 pairs of rudimentary ones. In succeeding moults the lateral eyes 

 become pedunculated, and the median eye disappears ; the ab- 

 domen grows longer, and its limbs are perfected. The Apus 

 quits the Qg% with fewer segments than it afterwards acquires. The 

 larval Sao has but three thoracic segments, but the number of these 

 increases with each successive moult, until the adult form is acquired ; 

 the new segments being developed between the thorax and abdomen. 

 Similar metamorphoses have been evinced by fossil specimens of the 

 tribolitic genus Ogygia, in which the additional segments were 

 developed at the hinder part of the abdomen. 



* ccxxxv. 



