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



placed within the same order, the Branchiopoda, though representing the type of an 

 anomalous suborder, the Phyllocarida. 



In order to facilitate a closer comparison of the Nebaliidse with the Copepoda, I sub- 

 j oin a cut of a male specimen of one of our most common marine forms of the Harpactoid 

 group, viz., Diosaccus tenuicornis (Claus). 



Homology of the Body-Divisions. — In examining the body of a Nebaliid, its general 

 resemblance to that of a Copepod, especially of the Harpactoid group, may at once be 

 recognised. But it is at the same time readily seen that there is in the Nebaliidae a 

 distinct division of the body which is only faintly indicated in the Copepoda, viz., the 

 trunk, or, as it is generally termed, the thorax. What is described as thorax in the 

 Copepoda does not at all answer to the thorax in the higher Crustacea, but undoubtedly 

 is homologous with the anterior part of the "abdomen" in these Crustacea, or the divi- 

 sion in the Nebaliidse described above as the pleon, whereas the so-called abdomen in the 

 Copepoda evidently answers to only the posterior part of the abdomen in the higher 

 Crustacea or the division in the Nebaliidte succeeding the pleon, and described above 

 as the tail. This is especially distinctly seen in the above described form, Paranebalia 

 longi'pes (PI. I. fig. 1 ; PI. II. fig. 1), where the latter division is very sharply marked off 

 from the pleon, both exhibiting a form very similar to that in the Copepoda, and, more- 

 over, quite agreeing in function, since the tail here evidently admits of being moved as a 

 whole upon the pleon, in the very same manner as in the Copepoda. A closer com- 

 parison between the Nebaliidas and Copepoda thus clearly shows that the terminology 

 generally adopted in describing the higher Crustacea has been wrongly applied as regards 

 the lower forms (Copepoda), since the divisions " thorax " and " abdomen " in the former 

 do not answer to the similarly named divisions in the latter. This misapprehension may 

 indeed have been the cause why the affinity of Nebalia to the Copepoda has not been re- 

 cognised. Thus, in order to explain the supposed abnormal number of segments in the 

 " abdomen " of Nebalia, Professor Claus has set forth an hypothesis, wdiich seems to me 

 very unreasonable, viz., that the two last segments together with the caudal rami in 

 Nebalia answer to the telson in the Podophthalmia, which latter part, he suggests, has 

 been originally formed by several segments. The fact is, however, that the so-called 

 abdomen in Nebalia does not show any similarity at all to that division in the higher 

 Crustacea, whereas it is constructed upon the very same type as in the Copepoda, the 

 number of segments being in full accordance with that found in a great number of these 

 Crustacea, admitting the above given explanation of the homology of the body-divisions 

 in both. As to the limit between the two divisions in the Nebaliidas, described above as 

 pleon and tail, it should be remembered that the first segment of the latter division, 

 properly speaking, answers to the segment in the Copepoda generally described as the 

 last thoracic segment, but which in most of the forms evidently has a much closer relation 

 to the succeeding division, the tail, or, as it is wrongly termed, the abdomen. 



