NEPHRIDIA AND BODY-CAVITY OF DECAPOD CRUSTACEA. 417 



tains the nerve-cord. These two sinuses are generally sepa- 

 rated by masses of muscle, but they communicate at intervals 

 by means of lateral sinuses. 



Theoretical Considerations. — We may seek for an ex- 

 planation of the relations above described by comparing them 

 with the condition of the body-cavity, as described by Sedg- 

 wick (No. 19) and von Kennel (No. 12), during the develop- 

 ment of Peripatus. Briefly stated, the course of events in 

 the latter animal is as follows : — The hollow mesoblastic 

 somites which form in the ventral bands of mesoderm divide 

 into two portions, which are distinguished as dorsal and 

 lateral. The lateral portions of the somites are transformed 

 into nephridia, whilst the cavity of the lateral sinus of the 

 body originates as a hollowing out in their thickened external 

 walls. The dorsal portions of the somites enlarge upwards, 

 but those of opposite sides do not meet in the dorsal middle 

 line. A space is left between them, which becomes the cavity 

 of the heart. At this stage the dorsal portions of the somites 

 may be said to have attained their maximum state of develop- 

 ment. By the diminution of their cavities they then cause the 

 formation of spaces external to themselves, which become lined 

 by wandering mesoderm-cells, and form the pericardial chamber 

 and central body-cavity of the adult. These two cavities 

 are separated by the pericardial septum, which is apparently 

 formed in a more direct way from the walls of the diminishing 

 somites. In certain regions of the body the remnants of the 

 dorsal portions of the somites persist, according to Sedgwick, 

 as the external membrane of the genital glands ; whilst, accord- 

 ing to V. Kennel, they give rise both to the internal and ex- 

 ternal cells of these organs. The condition in the adult Peri- 

 patus is described by Sedgwick (No. 19, pt. 4, p. 385) as 

 follows : — '' The haemocoele is divided into five main chambers : 

 (1) the central compartment of the body-cavity; (2) the 

 heart; (3) the pericardial cavity; (4) the two lateral com- 

 partments or lateral sinuses (in which the nerve-cord and 

 salivary glands lie). In addition to these there are the 



