4 i8 



THE CIRRIPEDS, TQNICATES AND ECHINODERMS. 



lacunal, follows well marked channels. The remarkable part of it is that the blood 

 currents, as he states, " Do not continue to flow for any length of time in the one 



direction. At one period they may be flowing as indicated They then 



suddenly slacken and reverse and stream for a time in exactly the opposite course." 

 (p. 21.) In this particular, therefore, the circulation is astonishingly like the 

 well known reversing circulation of the tunicates. 



The Eyes. In the tunicates the lateral eyes are absent, as they are in the 

 adult stages of cirripeds and all other acraniates. The parietal eye, however, may 

 be retained, and in some forms, as Salpa, it may be present and even well 

 developed in the adult stages, resembling in a very striking manner the nauplius 

 eye, or trioculate median ocellus of primitive crustaceans. 



The details of its early development are obscure, especially the manner in 

 which the several retinas are infolded and become lodged on the roof of the neural 



FIG. 287. The forebrain, ventral cord, and ocelli of Cyclosalpa, chain form, adult. A, Seen from the neural sur- 

 face; B, in cross-section; C, in longitudinal section. (After Metcalf, slightly modified.) 



tube. Judging from the structure of the eye, and from what is known of the de- 

 velopment of the medullary tube, the tunicate parietal eye appears to develop 

 in essentially the same manner as the median eye of Branchipus. In other words, 

 it is to be regarded as a true parietal eye, consisting of two pairs of ocelli, the 

 retinas of which have become loosely united to form the walls of a common 

 unpaired vesicle, opening into the cavity of the forebrain, and forming a part 

 of its roof. (Fig. 287.) The eye is innervated by two principal nerves, , that 

 arise from a dorsal ganglionic mass, probably representing the forebrain plus the 

 optic ganglia. 



The Old Mouth and the New. When the medullary plate, which represents 

 the entire brain and nerve cord of the ancestral arthropod, was infolded, the parietal 

 eye and primitive stomodaeum were carried in with it, and when the medullary 

 tube finally closed, the primitive mouth and the stomodaeum were permanently 

 shut off from the exterior; but they retained their original structure and relations 

 essentially unchanged, for the stomodaeum persisting as the subneural gland and its 

 duct, opens at its outer end, through the floor of the embryonic brain into the 

 rudimentary cerebral vesicle; while the inner end still opens into the enteron as 

 the so-called dorsal tubercle. (Figs. 284-286.) 



