THE TUNICATES. 



415 



formed that became the haemostoma of the tunicates, enteropneusta, and other 

 acraniates. 



It is often assumed that a sessile or parasitic mode of life is the initial cause 

 of degeneration. The various anatomical peculiarities common to the copepods, 

 cirripeds, and acraniates do not bear out this conclusion. The fact that in these 

 diverse sub-phyla we see the same shifting of cephalic appendages to the haemal 

 side, the same cephalic outgrowths, and the same degeneration of the neuro- 

 muscular organs, indicates that there are certain initial defects, or peculiarities 

 of germinal material, common to the whole group, that is the underlying cause of 

 a defective organization, and the defective organization is in every case of such 

 a nature that a sessile, or parasitic, or vegetative mode of life is the only one 

 possible. 



_e peopiT.^'^'^''*' ^^ CalLcju&, 



Fig. 283. — Lepeoptheirus. D, Hasmal, E, neural surface. F, G, Caligus. (From Scott, slightly modified.) 



II. The Tunicates. 



It was from a stock similar to that of the copepods and cirripeds, and one 

 dominated by the same fundamental defects in germinal material, that the 

 tunicates arose. They are built on the same general plan, pass through a similar 

 larval existence, and undergo a similar retrograde metamorphosis. But all this 

 is at first sight effectively disguised by the permanent closing of the old mouth, 

 and the opening of a new one on the haemal surface; by the absence of both the 

 chitenous and calcareous exoskeleton and appendages; and by the conspicuous 

 development of perforated gill sacs, and of the middle cord, or notochord. Like 

 the cirripeds they begin life with the same brave display of animal vigor, of well 

 developed brain, eyes, and locomotor organs that bespeak an efficient past and a 

 hopeful future, only to have them dwindle almost to extinction in a peaceful, 

 sedentary existence. 



The early stages of the tunicate embryo are essentially like those of a primi- 

 tive crustacean (Moina, Cetochiles, Balanus, or Palaemon). In both types there 

 is the same kind of cleavage; the same terminal proliferation or infolding to form 

 a telocoele; the same slipper-shaped medullary plate; the same middle cord or 



