MORPHOLOGY OF THE COPEPODA. 287 



the absent succeeding ring, must have become the origin of 

 the rudimentary (fifth) thoracic somite and its pair of feet. 



Now, whilst in the further course of development the 

 segmentation of the abdomen proceeds to its normal termi- 

 nation, the antennae and pairs of feet rcm.ain in one of the 

 last stages of development, and never attain to their complete 

 form. 



The morphological stage, therefore, of the form in question, 

 in respect of the articulation of the appendages, corresponds 

 to one of the latest stages of development ; whilst the absence 

 of the fourth thoracic ring, and corresponding pair of feet, 

 must be explained by reference to the differentiation being 

 interrupted at an early period. But the duplex characters of 

 the separate parts of the body remains a remarkable fact, and 

 1 cannot but express the notion that I may be describing a 

 form produced from two distinct species, in whose duplex 

 nature must at once be sought the cause of the deviations in 

 development. It is to be hoped that further investigations 

 may serve to solve this not uninteresting question. 



II. On the Structure o/Nicothoi: (figs. 3, 4, 5). 



Besides Audouin, and Milne-Edwards,'^ Kroyer,t Rathke,J 

 and Van Beneden § have contributed to our knowledge of 

 this Copepod, which is parasitic on the branchiae of Astacus 

 marinus. Although the above zoologists have studied the 

 subject at diff'erent periods, and to some extent under 

 different points of view, their observations collectively afford 

 a tolerably correct account of the structure, development, and 

 habits of this interesting parasite. At the same time there are 

 still some points, particularly with respect to the form and nature 

 of the oral organs, which, owing to the difficulty attending 

 the examination, have remained almost unnoticed, although 

 the importance of a knowledge of these organs for the proper 

 estimation of the systematic position of the animal is sufficiently 

 obvious. More recent examination, moreover, has shown that 

 even its structure has not been described in all respects ex- 

 actly as it is, and that that part of the subject is by no means 

 exhausted ; I am, therefore, induced to think that there is 

 some justification in my attempting to correct and complete 

 what has been already done in it. What has especially 

 induced me to draw the attention of naturalists again to the 



* 'Ann. d. Sc. nat.,' i, ser, torn. ix. 



f ' Naturhistorisk Tidiskrift,' Bd. ii. 



X ' Nov. Act./ torn. XX. 



§ 'Ann. d. Sc. nat.,' iii, Ser, torn. xiii. 



