242 THE VOYAGE OF H.M.S. CHALLENGER. 



exists as an obtuse process of the propodos, the two lying in juxtaposition, without having 

 the characteristic movement of a chela, and the basecphysis is longer than the limb. 



The posterior two pairs of pereiopoda are simple, having the seven joints visible, and 

 support a short basecphysis. 



The first pair of pleopoda is in a bud-like condition, but the four following pairs 

 are long, slender and biramose, the branches being membranous, flexile, and free from 

 hairs. 



The posterior pair of pleopoda is well developed, armed on the outer margin near the 

 distal extremity with a small tooth, and fringed on the inner with small hairs ; it reaches 

 beyond the extremity of the telson. 



PL XLVIII. fig. 3, represents the rhipidura of a specimen taken in the Western Pacific. 



Length, 10 mm. (0'4 in.). 



This form differs from the others in having the rostrum, which is as loDg as the 

 carapace, serrate towards the extremity, and it has no tooth on the dorsal crest. The 

 pleon is armed with a strong tooth, projecting posteriorly from the third somite, all 

 the other somites being smooth. The telson is long, with parallel sides, and becomes 

 suddenly acuminate distally ; it is fringed on the distal margin with four hairs on each 

 side of the central point, and each lateral margin is furnished with two small points or 

 teeth. The lateral branches of the rhipidura are scarcely as long as the telson. 



The branchiae (fig. 3br) are well developed. The three anterior pairs of pereiopoda 

 possess well-formed chelae, but shorter than the basecphysis. The posterior two pairs are 

 simple, but short and support a long basecphysis. The pleopoda are moderately 

 developed and well formed, and the whole animal is characteristic of a true Penseus, and 

 probably becomes mature in the next stage. 



PL XLVIII. fig. 4, represents what appears to be a mature but young form of some 

 such species as Penseus monodon, or, judging from the unequal length of the two fiagella 

 of the first pair of antennae, of Hemipenieus tomentosus. 



Its length is 6 mm. It was taken in the surf off Samboangan. 



Tracing the above series of young animals from Fritz Midler's and Claus's earlier 

 Protozoea stage, we find that each successive form corresponds with a higher stage of 

 development. In one or two instances, where there has been a deviation in relative size 

 compared with the degree of development, it may be assumed that the young of larger 

 species attain greater dimensions in their successive stages than those of smaller species. 

 Thus it appears that we may accept the history of the progressive development of the 

 young in a family as corresponding with that of a single genus, and if so it may be 

 accepted as being parallel with the development of a single species. 



Two links of importance are yet wanting : the one is that which connects the earliest 

 Protozoea form with Fritz Midler's Nauplius, and the other that which connects the 



