104 JOURNAL OF MARINE ZOOLOGY AND MICROSCOPY. 



hunger presses, but that there is something more seems argued by 

 the fact that each species is alwa}^ found with its own particular 

 host. If it were a question of protection only, would not the tube 

 of one worm be as good as that of another ? 



Another curious point about these worms is the great power 

 possessed of reparation of injuries. If in danger, it is the invariable 

 habit of P. propinqua, in common Avith many allied species, to break 

 up into two or three fragments in the hope of the head part being 

 able to escape under cover of the consequent confusion. If it be 

 able to do so, in a very short time a number of rudimentary somites 

 appear sprouting from the broken end, and these grow and increase 

 until .finally the full and complete size be once again attained. I 

 have seen individuals in all stages of this regeneration, but have 

 never seen the hinder or tail fragment of a ruptured worm reproduce 

 a fresh head, as has been recorded by some writers. Such however 

 would, I believe, be likely to occur were the fracture close to the 

 head, but usually it occurs much nearer to the tail than to the head, 

 and in such cases I feel sure that the tail portion dies. 



Study XIII. — The Tadpole Larvae of Ascidians. 



The Tunicates or Ascidians, may be either fixed or free ; simple 

 or colonial. The central type of the free is seen in the Appen- 

 dicularise, transparent tadpole-like animals that sport in the surface 

 waters of our seas, in profusion, at certain seasons. In size these are 

 extremely minute, the British species averaging only i-in. to I -in. 

 in length, tail included, while the length of lj-in. which a foreign 

 species reaches, is altogether exceptional and monstrous. In this 

 type, the tail is supported and strengthened by a firm central rod, 

 the notochord, believed to be a similar structure to the first sup- 

 porting stiff axis of the embryos of vertebrates, and which hi them 

 is subsequently usually obliterated by the encroaching growth of the 

 vertebral column. Another curious point is that two openings com- 

 municate between the pharynx and the exterior, and serve as gill 

 slits. 



These tiny ocean wanderers are never colonial, always free and 

 simple. The fixed Tunicates, the Ascidians proper, are, on the 

 contrary, very frequently colonial, or as some writers term it, com- 

 posite or compound. In such latter, there is usually a certain amount 

 ot federation, such for example in those where the anal apertures 

 of a circlet of individuals open into one common atrial or cloacal 

 chamber, to be expelled by a centrally place atrial opening. 



