92 



THE PINEAL ORGAN 



anus are brought near together at the anterior end of the body. The 

 dorsal surface or back of the animal is thus markedly reduced in length. 

 The adult animals of all these three classes are fixed and no sensory 

 organs are present, but the ova give rise to free-swimming larva; of the 

 trochophore type which in some species have a typical apical plate, bearing 

 a vertical tuft of cilia and lying over a cerebral ganglion. In close relation 

 with this, e.g. in the Phoronida, Phascolosoma (Podaxonia) (Fig. 58), 



Fig. 59. 



A — Pater ina. Simple ancestral fossil form of Brachiopod — enlarged ; lower 



Cambrian rocks, America. 

 B — Rhynchonella. Dorsal aspect of shell, showing foramen, /., in beak which 



transmitted the stalk. Fossil shell. Lower Cretaceous. 

 C — Young larva of Cistella, showing two eye-spots, three segments, and two 



bundles of sets. 

 D — Interior of dorsal valve of Cistella, which closely resembles the fossil types of 



Brachiopoda shown in A and B. The simpler species have presumably 



persisted with little change in their form or mode of development since the 



lower Cambrian period (see Fig. 324, p. 479). 



there are two pigmented eye-spots which lie right and left of the median 

 plane ventral to the plate. Bilateral eye-spots are also present in the 

 free-swimming larva of Cistella. When the larvae become fixed the eye- 

 spots disappear. A point of great interest with regard to the presence 

 of paired eyes of simple type, in the larvae of the Brachiopods is that the 

 more primitive forms of this class, judging from the characters of the 

 shell, have existed almost unchanged from the palaeozoic period represented 

 by the lower Cambrian rocks of America, e.g. the hinged bivalve brachiopod 

 Paterina, which is regarded as the ancestor of the living brachiopods 

 (Fig. 59). 



