CCELOM AND VASCULAR SYSTEM OF MOLLUSCA. 429 



previously existing coelora. Nevertheless the coelom still per- 

 sists in some parts of the Arthropod body quite separate from 

 the swollen blood-vascular system. It persists as the tubular 

 generative glands (perigonadium), and also as a system of 

 small spaces (lymph-system) in the connective tissue of Astacus 

 and of Limulus, and as the internal terminal vesicle of the 

 green glands and other nephridia present in various Arthropoda. 

 Professor Lankester stated that he had been led to this view 

 with regard to the vascular system and coelom of the Arthro- 

 poda by the results of his histological investigations on the 

 vascular system and connective tissues of Astacus and Liraulus, 

 and by the results obtained in his laboratory by Mr. Gulland 

 in studying the development of the nephridial ' coxal gland ' 

 of Limulus (already published^ with note by Professor Lan- 

 kester, in the ' Quart. Journ. Micr. Sci./ 1885, vol. xxv, 

 p. 515). He had also been led to this view by the attempt to 

 explain theoretically the origin of the peculiar structure of the 

 Arthropod's heart and blood-holding pericardium. 



'' The Arthropod^s heart and pericardium are absolutely 

 peculiar to the group, and characteristic of all its members, 

 even of Peripatus. The author had asked himself how the 

 existence of a tubular heart with paired valvular apertures in 

 each segment of the body, lying within a blood-holding sac, 

 could be explained. He conceived that it might best be ex- 

 plained by that tendency of the veins to dilate and to form 

 irregular large blood-sinuses, which on other grounds we have 

 reason to consider as a structural tendency of Arthropods. Each 

 pair of valvular apertures in the Arthropod's heart represents 

 a pair of distinct tubular veins, which in the ancestors of the 

 Arthropoda brought blood to the heart from the gills. These 

 veins have dilated, and their adjacent walls have been ab- 

 sorbed, so that we now have, instead of a series of veins, a 

 great continuous blood-sinus on each side of the heart or 

 dorsal vessel. 



" Capillaries of the finest dimensions were shown by Professor 

 Lankester to exist in certain parts of Astacus and of Limulus. 

 In studying these he had come across the remnants of coelom. 



