EARLY STAGES OF TEREBRATULINA. 31 



the scale-like structure so characteristic in later stages. Between this stage and the next, 

 tig. .3, the shell rapidly elongates, wliile the hinge margin remains nearly the same in 

 width ; this is also shown in the concentric lines of growth seen faintly on the surface, 

 indicating a rapid increase in the length of the shell, while no corresponding increase 

 takes place in the widening of the hinge margin. The peduncle is longer than the shell, 

 having distinct walls apparently enclosing a clear interspace, the end slightly dilating and 

 forming a pear-shaped adhering disk. The structure of the shell, of which more will be 

 said in subsequent pages, showed clearly the scale-like structure, with the ca)cal tubules 

 of the pallial lobes perforating it. The anterior margin of the pallial lobes gave rise to 

 seven setoe of variable lengths, all of them projecting forward. These setae, in nearly all 

 instances, were clothed with desmids, and were probably available in attracting sustenance 

 to the animal ; they resembled in structure the setfe of the adult, as tigured by Hancock, 

 and the longest of them was as long as the shell. The future position of the calcareous 

 loop was indicated by a strongly arched process midway the length of the shell, from 

 which sprang six short and stout cirri, all of them curving towards the mouth, which 

 occupied the centre of the base from which the cirri sprang. The digestive sac hung 

 from the mouth, and was twice as long as broad, havina; a stronar constriction in the cen- 

 tre, forming two chambers, the lowermost one being globular in shape, and having its 

 walls colored a light reddish-brown, this colored portion evidently indicating hepatic cells. 

 The cavity next the mouth indicates the stomach, while the lowermost cavity indicates 

 the future intestine, or cul de sac. The cirri moved frequently, and in various directions, 

 though generally performing a grasping motion, as if securing some bit of food, imitating 

 precisely the movement of the cirri in Polyzoa, and this resemblance was more complete 

 from the fact that the tentacles were densely clothed with cilia (fig. 4), and their move- 

 ments caused visible currents in the water. The two cavities of the diarestive sac were also 

 ciliated, and little pellets of food were seen rapidly circulating back and forth from one 

 cavity to the other. The cavities were alternately dilating and contracting, and assumed 

 the forms represented in figs. 5, 6 and 7. At this stage the loAver cavity of the digestive 

 sac was diverted to the larger valve, as represented in fig. 4. 



In this stage, and several succeeding stages, the outline of the shell is remarkably like that 

 of Linrjnhi, and this resemblance is more striking from the proportionately long peduncle. 



Tn another stage, numerous irregular shaped calcareous spiculse lined the outer margin 

 of the cirri, while the future position of the calcareous loop, or crura, was indicated by a 

 row of irregular shaped spiculns, the correct forms of which are given in fig. 15. 



Fig. 16 represents portions of two cirri more highly magnified, in which correct out- 

 lines are given of the spiculaa. It is to the presence of these calcareous particles that we 

 must account for a certain rigidity noticed in the texture of the cirri. In these slightly 

 advanced stages, the peduncle becomes much shorter in proportion to the length of the 

 shell, though the shell presents the same lingula-like form, diliering, however, from Xin^/H/rt 

 in the much greater length of the neural valve, which is always perforated and rostrated 

 from the outset. 



In all these stages the peduncle has very slight adhesion to the rock, or whatever sub- 

 stance it may be attached to, in this respect differing greatly from the adult, Avhich 

 often requires great force to detach it, frequently leaving the peduncle separated 

 from the body, and adhering to the rock. 



