NOTES. 355 



like one of the ring-shaped air-cushions one sees, on a very small scale. 

 Dr. Grobben describes the spermatoblastic cells of the testis and their 

 nuclear spindles ; but his account of the development of the spermatozoa 

 does not agree with my own observations, which, so far as they have 

 gone, lead me to infer that the annulate corpuscle of the spermatozoon 

 is the metamorphosed nucleus of the cell from which the spermatozoon is 

 developed. For want of material, however, I was unable to briag my 

 investigations to a satisfactory termination, and I speak with reserve. 



Note XIV., Chapter IV., p. 174. 

 THE MORPHOLOGY OP THE CRAYFISH. 



The founder of the morphology of the Crustacea, M. Milne Edwards, 

 counts the telson as a somite, and consequently considers that twenty- 

 one somites enter into the composition of the body in the Podoph- 

 thaliiiia. Moreover, he assigns the anterior seven somites to the head, the 

 middle seven to the thorax, and the hinder seven to the abdomen. 

 There is a tempting aspect of symmetry about this arrangement ; but as 

 to the limits of the head, the natural line of demarcation between it and 

 the thorax seems to me to be so clearly indicated between the somite 

 which bears the second maxillae and that which carries the first maxiUi- 

 pedes in the Crustacea, and between the homologous somites in Insects, that 

 I have no hesitation in retaining the grouping which I have for many years 

 adopted. The exact nature of the telson needs to be elucidated, but I can 

 find no ground for regarding it as the homologue of a single somite. 



It will be observed that these differences of opinion turn upon ques- 

 tions of grouping and nomenclature. It would make no difference to 

 the general argument if it were admitted that the whole body consists 

 of twenty-one somites and the head of seven. 



Note XV., Chapter IV., p. 199. 

 THE HISTOLOGY OF THE CRAYFISH. 

 In dealing with the histology of the crayfish I have been obliged to 

 content myself with stating the facts as they appear to me. The discus- 

 sion of the interpretations put upon these facts by other observers, espe- 

 cially in the case of those tissues, such as muscle, on which there is as 

 yet no complete agreement even as to matters of observation, would 

 require a whole treatise to itself. 



