420 



THE CIRRIPEDS, TUNICATES AND ECHINODERMS. 



time, living tissue. In Corunula, the shell contains large spaces which stand in 

 direct communication with the central cavity of the test, and in which is imbedded 

 that part of the ovary that gives rise to the eggs. (Gruvel, p. 368.) 



The remarkable structure of the shell in the cirripeds recalls that of Limulus 

 and Pteraspis (Figs. 196-204.) At the same time the invasion of its matrix by 

 isolated hypodermal cells, by pyramidal ingrowths of the mantle, and by ovarian 

 tubules, is comparable respectively with the presence of the test cells, vascular in- 

 growths, and the so-called "kalymmocytes," or egg follicle cells, in the mantle of 



A 'm 



FIG. 288. Sections of the shell of cirripeds. A, Pachylasma; B, Balanus. (Afte; Gruvel.) 



the tunicates. The principal difference, therefore, between the exoskeleton of the 

 tunicates and the complicated ectodermal skeleton of Limulus, the pteraspids, and 

 the cirripeds apparently lies in the different chemical composition of the non- 

 cellular matrix. 



Tunicates and Cirripeds. Summary. 



It is unnecessary to carry our comparison into further detail. The structure 

 and mode of growth of the tunicates justify the conclusion that they are descended 

 from animals in which the salient characteristics of primitive arthropods were 

 fully established; in fact, from that particular subdivision of the arthropods to 

 which the cirripeds and copepods belong. Here some inherent defects in the 

 germinal material impose on both ancestors and descendants those peculiarities 

 of structure and mode of life that are common to both. 



The tunicates resemble the cirripeds: i. In the structure of their larvae, in 

 their mode of attachment, and in their subsequent revolution, degeneration, and 

 metamorphosis. 2. In the investment of the body by a huge fold of the skin, or 

 mantle, that encloses an atrial, or peribranchial, chamber. 3. In the outgrowth 

 that arises from the haemal surface of the head to form the stalk or pedicle. 4. In 

 the occurrence of a reversing circulation. 5. In the presence of a parietal, tri- 

 occellate eye. 6. In the presence of an exoskeleton, which in the one, consists of a 

 calcined, chitenous secretion of the ectoderm; in the other of a substance resem- 

 bling cellulose; each has a complex and unusual structure, but one that is essentially 

 the same in both. 7. In the tunicates, the enteric pouches have perforated the 

 body wall; the old mouth is permanently closed, and a new one has opened on the 



