i 4 6 COMPARATIVE ANATOMY 



abdominal region. In the next stage, this band is divided up 

 into a number of paired groups corresponding with the external 

 segmentation of the embryo, and in the abdominal region each 

 group acquires a lumen, so that there is a transitory stage in 

 which the mesoblast of the abdomen is divided up into a series 

 of segmentally-arranged, paired pouches, bearing a close 

 resemblance to the mesoblastic somites of the Annelida. This 

 condition, however, is not of long duration. The walls of the 

 pouches are differentiated to form the flexor and extensor 

 muscles of the abdomen, and all trace of their cavities is lost. 

 It is probable that, while they exist, the mesoblastic pouches 

 are representative of the somites of worms, and their cavities 

 are to be regarded as coelomic cavities. Unfortunately we 

 have no definite information about the development of the 

 gonads in Astacus. They make their appearance late in 

 embryonic life, and so far as our information goes, they do 

 not appear to have any connection with the transitory meso- 

 blastic pouches which have just been described. This, it must 

 be admitted, tells against the coelomic nature of the cavities 

 of the pouches, for the cavities of the adult gonads must, 

 according to the principles already laid down (p. 52), be 

 regarded as the representatives of the true coelom of the adult. 

 Nor does the development of the excretory organs of the 

 crayfish throw much light on the question of the coelom. The 

 green glands appear, at an early stage, as a pair of epiblastic 

 invaginations at the bases of the second antennae, and in 

 subsequent stages there is no trace of their entering into 

 connection with any mesoblastic cavity or structure which 

 could be regarded as coelomic. On the other hand, the green 

 gland of the lobster is described as being formed as an 

 epiblastic invagination which meets and communicates with a 

 mesoblastic vesicle, the latter becoming the end-sac of the 

 adult organ, while the epiblastic tube forms the labyrinth, 

 bladder, and duct. Larval excretory organs have not been 

 described in the crayfish, but in many decapod Crustacea a pair 

 of such organs is formed at the bases of the second maxillae. 



A late embryonic stage is represented in fig. 35. The ventral 

 flexure is pronounced, the head and telson being in contact. 

 The general organisation of the body can easily be followed in 

 the figure, and it should be noticed that all the characteristic 

 anatomical features of the adult are represented. The 



