486 TUNICATES 



tadpole ; and then, if it is a typical Ascidian, it settles down and 

 undergoes " retrograde metamorphosis ". 



Let us form a picture of the larval or " tadpole " Ascidian 

 (Plate XXXVI., A). It has a minute oval anterior region, 

 and a delicate vibratory tail by which it swims. Running 

 along the axis of the tail there is a soft rod — the notochord 

 — by the side of which there are strong muscle-bands. Above 

 this, along the dorsal median line, just below the surface, there 

 is the tubular nervous system, that expands anteriorly into a 

 " brain " or cerebral vesicle. In connection with the roof of the 

 latter there is an unpaired " brain-eye " — an intra-cerebral eye. 

 that does not get out of the vesicle. On the floor there is an 

 otolith. An intucking of the outer embryonic layer (the 

 ectoderm) forms the mouth, and in a very complex manner — 

 far beyond our present level of description — two or more gill- 

 slits are formed, opening from the pharynx through the atrial 

 cavity to the exterior. Of the essential chordate characters, 

 the larval Ascidian has five: (i) a dorsal tubular nervous 

 system (very delicate posteriorly), (2) a supporting rod or 

 notochord (confined to the tail-region), (3) gill-clefts, (4) a 

 ventral heart, (5) a brain-eye. There is no definite segmenta- 

 tion of the body, and there is no trace of nephridia. 



But the definitely Chordate larva is the creature of a day. 

 It swims about for a few hours, perhaps for a day, and then it 

 attaches itself to some stone or seaweed by one or more of three 

 glandular papillae borne on the front of the head below the 

 mouth. (Plate XXXVI., B.) 



With great rapidity degeneration sets in. The tail — with its 

 notochord, nerve-tube, and muscles — is absorbed (and to a 

 slight extent lost in shreds). Wandering amoeboid cells or 

 phagocytes, which are present in all animals except Thread- 

 worms and Lancelets, help in the absorption of the tail, and mi- 

 grate into the anterior body. A test grows rapidly, and 

 replaces the attaching papillae. By a remarkable inequality of 

 growth the posterior part of the body is made to rotate through 

 about 180 , so that the anus and exhalant aperture are shunted 

 to the free extremity. (Plate XXXVI., C.) Thus in a few 

 hours an unmistakable Chordate becomes an enigmatical 

 nondescript. 



The Tunicate's life-history illustrates rapidity of develop- 



