116 ARACHNID A. 



which makes this point of resemblance appear of less importance), 

 so that this feature may perhaps be inherited from a common 

 ancestral form. A less important agreement with the genital glands 

 of Limulus is afforded by the corresponding tubular network of 

 genital glands in the Scorpiones. 



The coxal glands of the Arachnida, derived from the mesoderm, 

 may, according to our present knowledge, be assumed with consider- 

 able certainty to be nephridia, and are comparable with the organs 

 which, in Limulus, occupy a corresponding position. These glands 

 cannot be fully homologised with the antennal and shell-glands of 

 the Crustacea, since these latter differ somewhat from them in. 

 position, i.e., belong to other segments. The nephridia that were 

 present in every segment in the ancestral form have undergone great 

 reduction, and the remnants are retained by their descendants in 

 different segments, a feature probably connected with the varying 

 form of the adult body in the different groups. We need hardly 

 point out that the possession of coxal glands (especially strongly 

 developed in youth) is a further distinction between the Arachnida 

 and the Insecta, the latter not possessing any glands which in 

 their development and position could be compared with the nephridia 

 of the ancestral form. 



The Arachnid coxal glands arise from the mesoderm, the condition 

 of which during embryonic development is a point of special im- 

 portance. "While, in the Insecta, the primitive segments are early 

 subjected to change, in the Arachnida, they grow forward dorsally, 

 and only undergo disintegration at a time when the dorsal heart is 

 formed from them. The coelom, which disappears very early in 

 the Insecta, is long retained in the Arachnida. This, which in itself 

 is a primitive condition, further determines a greater simplicity in 

 the rudiment of the heart, perhaps also in that of the coxal glands 

 (nephridia), and probably also of the genital glands. The conditions 

 thus produced recall those in the Annelida more than those in the 

 remaining Arthropoda. 



It appears open to question whether much stress should he laid on the agree- 

 ment existing between the cleavage, and the formation of the germinal layers 

 and of the first rudiments of the organs in the Arachnida and the processes 

 described for the Crustacea, or whether these should be explained by a certain 

 similarity prevailing in these processes throughout the Arthropoda. This has 

 already been pointed out in individual cases. It must remain equally doubtful 

 whether the youngest stage of the germ-band in the Scorpiones, which has been 

 compared with a certain ontogenetic stage in the Trilobites (p. 6), is of special 

 importance in this connection. It can hardly be doubted, from all that has 



