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



that in the other forms, and also in the Branchiura the nervous cord is constructed upon 

 a quite different type. 



The intestine in Nebalia is markedly distinguished by the presence of a chitinous 

 visceral skeleton never found in any Branchiopoda ; but this skeleton does not show 

 any resemblance to that found in the Podophthalmia, whereas its similarity to that 

 found in the Amphipoda is unmistakable. With the latter Crustacea Nebalia also 

 apparently agrees in the two pairs of elongate cseca arising from the gastric part and 

 accompanying the intestine in the greater part of its length, and more especially in the 

 presence of another pair of elongate csecal appendages originating from the terminal part 

 of the intestine and running anteriorly along its dorsal side. On the other hand, the 

 occurence of two short curved cseca, projecting forwards within the head, is a character 

 pointing rather more to the Branchiopoda, in some of which, as in the genus Daphnia, 

 we find two quite similar curved caeca arising from the anterior part of the intestine. 



As to the structure of the heart the genus Nebalia may equally well be compared 

 with the Branchiopoda as with other Crustacea, since the structure of this organ in the 

 former is very variable, being in some forms quite short and sac-like with only a single 

 pair of lateral venous fissures, in other forms more or less elongate with a varying number 

 of such fissures. As in the Branchiopoda the vascular system is imperfectly developed, 

 though a pair of short arteries may be traced at each end of the heart. 



The generative organs in both sexes, as in most of the Branchiopoda, are rather 

 similar and very little complicated in structure, forming merely a pair of simple 

 cylindrical tubes extending along the sides of the intestine, and opening by a short 

 excretory duct on the ventral surface of the last segment of the trunk. In the 

 Podophthalmia these organs are, as is well known, much more complicated, and, 

 moreover, always exhibit a very marked difference in their structure in the two 

 sexes. 



Development. — The development of Nebalia cannot in my judgment be adduced as 

 showing any close affinity between this form and the Decapoda, as suggested by 

 Metschnikoff ; and this has partly also been admitted by Professor Claus, in so far, at 

 least, that he has pointed out that the stage of the Nebalia-embrjo, termed by 

 Metschnikoff the Zoea-stage, and on which this author chiefly bases his suggestion of 

 the decapodous nature of Nebalia, does not in reality correspond to that stage in the 

 Podophthalmia, but more properly to the earliest stage of the Cyclops-fovm in the 

 Copepoda. Neither can I find that the supposed agreement between the development 

 of Nebalia and that of Mysis points to any true consanguinity between these genera. 

 As is well known, we find a very similar direct development also in a great number of 

 other Crustacea not at all belonging to the Podojththahnia, as in the Cumacea and 

 Isopoda, and even in the Phyllopoda I have recently had an opportunity of stating an 

 instance of a quite direct development, without any metamorphosis, and agreeing, more- 



