XVII DEVELOPMENT 479 



runs a blood-vessel, which doubtless serves to nourish the gland, 

 the outer surface of which is bathed in the perivisceral fluid. 

 Every gradation can be found between the ripe generative cell 

 and the ordinary cell lining the body cavity. When the ova 

 and spermatozoa are ripe they fall off from the ovary and testis 

 respectively into the body cavity, thence they are conveyed to 

 the exterior through the nephridia. The ova in certain genera, 

 such as Argiope^ Cistella^ and Thecidium, develop in brood- 

 pouches which are either lateral or median involutions of the 

 body wall in the neighbourhood of the external opening of 

 the nephridia ; they are probably fertilised there by spermatozoa 

 carried from other individuals in the stream of water which 

 flows into the shell. In other species the ova are thrown out 

 into the open sea, and their chances of meeting with a sperma- 

 tozoon is much increased by the gregarious habits of their sessile 

 parents, for as a rule considerable numbers of a given species 

 are found in the same locality. 



The Embryology 



We owe what little we know of the Embryology of the group 

 chiefly to Kowalevsky,^ Lacaze-Duthiers,^ and Morse. ^ The 

 Russian naturalist worked on Clstella (^Argiope) iieapolitana, the 

 French on Thecidiitm^ and the American chiefly on Terehi-atidina. 



Although this is not known with any certainty, it seems 

 probable that the eggs of Brachiopods are fertilised after they 

 have been laid, and not whilst in the body of the mother. The 

 spermatozoa are doubtless cast out into the sea by the male, 

 and carried to the female by the currents set up by the cilia 

 clothing the tentacles. 



In Tliecidium, Clstella^ and Argiope the first stages of devel- 

 opment, up to the completion of the larva, take place in brood- 

 pouches ; in Terehratulina the eggs pass out of the shell of the 

 mother and hang in spermaceti-white clusters from her setae 

 and on surrounding objects. In the course of a few hours they 

 become ciliated and swim about freely. The brood-pouch in 



1 Development of the Brachiopoda, 1873 (Russian). 



2 "Histoire de la Tliecidie," Ann. d. Sci. Nat, S6r. 4, vol. xv., 1861. 



3 " On the Early Stages of Terebratulina septentrionalis," ^fem. Boston Soc. 

 Nat. Hist., vol. ii., 1869. "On the Development of Terebratulina," Ibid. vol. 

 iii., 1873. 



