C(ELOM OF PERIPA TVS 



vided with their own proper " coelomoducts," being excretory 

 modifications of the primary exits or ducts of the coelom, which 

 served in the ancestral coelomocoelous animal as exits for genital 

 products. 



Whilst the dorsal divisions of the coelomic sacs of Peripatus 

 are moving upwards towards the mid-line of the back, a space 



Fio. 13. TRANSVERSE SECTIONS SHOWING THE BREAKING UP OF THE CCELOJI, AND THE 



DEVELOPMENT OF THE H/EMOCCEL IN PERIPATUS. 



A, section of a young embryo, in which the only cavities present are 1, the gut or 

 metenteron, and 2, the cculom in the form of a pair of pouches. (in each segment) derived 

 from the wall of the primitive archenteron. B, section of a later embryo showing the 

 division of the coelom on each side into a dorsal and a ventral cavity (2, 2), and the 

 appearance of the hsemoccel as three longitudinal cavities (3, 3, 3). C, section of a later 

 embryo ; the dorsal cavities of the coelom have migrated to the dorsal mid-line ; the 

 ventral sacs acquire such an opening to the exterior. D, section of a still later embryo. 

 The dorsal portions of coelom (2) become the gonads (gonocoels) ; the ventral portions 

 <2') become urocoels with end -sacs (the so-called segmented organs usually, but errone- 

 ously identified with " nephridia "). The Iwemocoel shows a division into several com- 

 partments ; the heart (3') has made its appearance. The nerve cords (4), already visible 

 in C, are well developed, and portions of the slime-glands (5) are seen in section. (After 

 Sedgwick, from Sedgwick's Text-book of Zoology.) 



begins to make its appearance between the body-wall and the gut- 

 wall, and rapidly increases in volume (Fig. 13, B, 3). This is the 

 blood-space or hsemocoel. It is of very great importance that we 

 should have minute and repeated examination of the development of 

 this space in various Arthropoda, since light will thereby be thrown 

 on the primitive lines of historic development of the blood-vascular 

 system. From Mr. Sedgwick's description of the origin and 



