48o 



RECENT BRACHIOPODA 



is median, in the convex lower shell, in CisteUa it is paired, and 

 arises by the pushing in of the lateral walls of the body in the 

 reo-ion ^ust behind the horse-shoe-shaped tentacular arms ; the 

 renal ducts, which also serve as oviducts, open into these lateral 

 recesses. 



In the female Thecidiurn (Fig. 317) the two median tentacles 

 which lie just behind the mouth are enlarged and their ends 

 somewhat swollen ; they are bent l)ack into the brood-pouch, 

 and to them the numerous larvae are attached Ijy a short fila- 

 ment inserted into the second of the four segments into which 

 the larva is divided. In Cistella a similar filament attaches the 

 larvae to the walls of the brood sac ; thus tliey are secured from 



Fig. 317. — Brood-poucli of Thecidiurn 

 medlterraneum. (After Lacaze- 

 Duthiers.) Part of the wall of the 

 l">ouch has been removed to show 

 the clusters of larvae. 



1. Mouth, overhung by lijj. 



2. One of the two median ten- 

 tacles which are enlarged and modi- 

 fied to bear the larvae. 



3. Wall of brood - pouch into 

 which the median tentacles are 

 folded. 



4. Larva attaclied to the swollen 

 end of the tentacles. 



being washed away by the currents constantly flowing through 

 the mantle cavity of the mother. 



In Cistella the larva consists at first of two segments, but 

 the anterior one divides into two, so that in the free swimming 

 larva we find three segments, the hindermost somewhat longer 

 and narrower than the others and destined to form the stalk. 

 About the time of the appearance of the second segment four 

 red eye-spots arise in the anterior segment, which tends to l)e- 

 come constricted off from the others, and may now be termed the 

 head. It gradually becomes somewhat umbrella-shaped, develops 

 cilia all over its surface and a special ring of large cilia round 

 its edge. 



In the meantime the second or mantle segment has grown 

 down and enveloped the stalk, and four bundles of setae have 



