178 



CRUSTACEA. 



such a manner that the ventral precedes the dorsal fusion. This leads us to the 

 condition observed in the Decapoda. 



In Astacus, the first rudiment of the heart is to be recognised as an accumula- 

 tion of mesoderm-cells in the most posterior part of the embryonic disc (Fig. 63 

 B, h, p. 131), and thus behind the point from which the ventrally curved 

 thoraco-abdominal region arises. In sections it can be seen that these mesoderm- 

 cells unite to form a transverse plate which becomes opposed to the ectoderm on 

 each side. The cavity found between the ectoderm and the plate of cardioblasts- 

 is the future lumen of the heart. This plate already shows slight pulsation, in 

 which the ectoderm passively participates, before it curves round dorsally to- 

 form a tube (Reichenbach). A similar condition is described by Lebedinsky 

 for Eriphia (No. 57). 



The heart of Mysis develops in a manner analogous with that of Oniscus. 

 The heart here arises as a cavity in a kind of 

 dorsal mesentery formed by the fusion of the 

 lateral edges of the somatic mesoderm. Its- 

 formation proceeds from behind forward. 



As the primitive form of Crustacean 

 heart we must assume a long dorsal vessel 

 provided with numerous segmentally- 

 arranged pairs of ostia; such a heart 

 has been retained in the Branchiopoda 

 (Fig. 95, h, p. 202), and also occurs in 

 the Stomatopoda. The short, sac-like 

 heart of the Copepoda and Cladocera 

 is a degenerate form of this elongate 

 type. Such degeneration may lead in 

 small Entomostraca to the entire dis- 

 appearance of the heart (many Copepoda 

 and Ostracoda). In the same way the 

 short sac-like heart of the Decapoda is 

 probably to be derived from an elongate 

 type of heart, such as is found in the 

 Stomatopoda and Leptostraca. It seems 

 probable, from Claus's researches on the vascular system of the 

 Stomatopodan larvae with regard to the origin of the arteries, that 

 the heart of the Decapoda corresponds to the most anterior portion 

 of that of the Stomatopoda. 



Fig. 87. — Dorsal aspect of a pos- 

 terior segment in the body of a 

 young Branchipus larva, showing 

 the development of the heart 

 (after Claus). c, cardioblasts ; 

 h, cardiac cavity ; ms, mesoderm- 

 somites ; os, rudiments of the 

 ostia. 



H. Glands. 



Two pairs of glands which occur in the Crustacea must be 



regarded as modified segmental organs. These are the antennal 



gland (green gland) and the shell-gland. Reichenbach (No. 65) 



and Ischikawa (No. 51) have maintained that the former arises as 



