78 



BRACHIOPODA. 



rudiments of the hinge-teeth develop. These already occupy the 

 position they retain throughout life, lying at either side of the 

 original hinge-aperture, and thus, later, near the deltidium. When 

 the hinge -line, at a later stage, lengthens out laterally over the 

 teeth, as is specially the case in Orthis, Spirifer, and Strophomena, 

 this is a consequence of secondary processes of growth. 



ds 



Fig. 40.— Development of the pseudodeltidium in Thecidmm mediterraneum (after Beecher). 

 A, metamorphosed larva of Thecidium with the first rudiments of the shell (after Kowa- 

 levsky). B, adult Thecidmm, seen from the dorsal side. C, the same, seen in profile. 

 ds, dorsal shell-valve ; k, cephalic section ; p, pseudodeltidium ; s, hinge-line ; st, peduncle ; 

 us, ventral shell-valve. 



IV. General Considerations. 



A serious obstacle to our comprehension of the pelagic Brachio- 

 podan larva, as represented typically by the Argiope larva, is to be 

 found in the circumstance that the oral and anal apertures are here 

 wanting, their absence making the orientation of the body-surfaces 

 and segments extremely difficult and uncertain. We are inclined 

 to homologise the ciliated ring at the edge of the cephalic section 

 with the pre -oral ciliated band of the Annelidan Trochophore, 

 although Kowalevsky's observation as to the rise of the oesophagus 

 (p. 69) seems to contradict this view. The position of the neural 

 plate would be indicated by the eye-spots (and in Terebratulina, 

 Fig. 31 B and 0, by a long ciliated tuft) on the cephalic pole of 

 the larva. The anterior section of the body would thus possess 

 characters in common with the Trochophore, and would be com- 

 parable to the cephalic lobe of the Actinotrocha. 



Tn considering the significance of the posterior section of the 

 body, we must institute comparisons with the larvae of the Bryozoa 

 and of Phoronis. There can be no doubt that the point at which 

 attachment takes place is identical in the Bryozoa and Brachiopoda, 

 and corresponds to the posterior end of the body in the adult 

 I'/">roni8. The fixation of the body is accomplished by means of 

 the posterior pole of the body in the Brachiopodan larva; the pedal 



