1887.] 



ON CYCLESTHERIA HISLOPI. 



21 



c. The Trunk 



As representing tbis third principal division, I regard the 

 whole remaining movable part of the body, with the exception 

 of the last segment only. To transfer the limit between the 

 trunk and the tail farther on anteriorly, as proposed by some 

 naturalists, I find quite inappropriate and unwarrantable. True, 

 the female genital orifices in this, as in most other forms of 

 the present group of Phyllopoda, are located at the base of the 

 eleventh pair of legs, and in Branchipiis and Artemia there are 

 exactly eleven pedigerous segments in front of the tail, thus 

 apparently corroborating the supposition. that the part posterior 

 to the eleventh pair of legs should also in other Phyllopoda be 

 regarded as caudal. But such an assumption cannot, in my 

 opinion, be maintained, since there is another form of the Branchi- 

 podidæ, Polyartemia, in which the pedigerous segments belonging 

 to the trunk are far more numerous, amounting to no less than 

 nineteen. Moreover, there is in the present group of Phyllopoda 

 no difference whatever to be observed, either in the form of the 

 segments or in the structure of the limbs, between the anterior 

 and the posterior part of this region. 



In the extent I assign to this division of the body, it is 

 composed of sixteen well-defined segments, each of which bears a 

 pair of foliaceous legs. As to the form of the present division, 

 it is råtner narr o w cylindrical, its posterior part curving as a 

 rule more or less abruptly downwards. Between the upper side 

 of the trunk and the dorsal edge of the shell in adult females, 

 there is always a roomy space, w 7 hich serves for the recep- 

 tion of the ova during development, thus representing a so- 

 called incubatory cavity, or „matrix", which at last becomes 

 nearly filled up with the young brood (see Pl. I, fig. 3, Pl. 

 II, fig. 1). The eight posterior segments are each provided with 

 a lamellar dorsal process, curving more or less distinctly forward 

 and edged with slender posteriorly pointing setæ. These processes r 

 which undoubtedly serve for closing the incubatory cavity behind 

 and thus prevent the young from escaping before being fully 

 developed, are placed transversely to the segments and increase 



