48o 



RECENT BRACHIOPODA 



CHAP. 



Thecidium is median, in the convex lower shell, in Cistella it is 

 paired, and arises by the pushing in of the lateral walls of the 

 bod}^ in the region jast behind the horse-shoe-shaped tentacular 

 arms ; the renal ducts, which also serve as oviducts, open into 

 these lateral recesses. 



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

 Avhich lie just behind the mouth are enlarged and their ends 

 somewhat swollen ; they are bent back into the brood-pouch, 

 and to them the numerous larvae are attached by 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 they are secured from 



Fig. 317. — Brood-pouch of Thecidium, 

 rnediterraneum . (After Lacaze- 

 Duthiers.) Part of the wall of the 

 pouch has been removed to show 

 the clusters of larvae. 



1. Mouth, overhung by lip. 



2. One of the two median ten- 

 tacles which are enlarged and 

 modified to bear the larvae. 



3. Wall of brood -pouch into 

 which the median tentacles are 

 folded. 



4. Larva attached 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 be- 

 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 



