ASCIDIANS OF GUERNSEY. 



201 



most interesting-. This food consists of minute organisms 

 floating in the water, and these are strained and filtered out of 

 it by means of a circle of hair-like tentacles, which may be 

 simple or branched, situated within the branchial orifice. 

 Behind this circle there are two closely placed parallel ridges 

 forming a groove, the sides of which are richly ciliated. This 

 groove is filled with a sticky substance secreted by a long, 

 rod-like gland called the " endostyle," lying on the ventral 

 side of the branchial sac. The food particles which have 

 become entangled in the mucus are swept by ciliary action 

 into another canal, called the "dorsal lamina," lying opposite 

 the endostyle and communicating with the stomach. This 

 dorsal lamina may have tags on the margin that in some 

 Ascidians become long processes called " languets." Frecal 

 matter is ejected with the waste or filtered water. 



While speaking about the branchial sac, blood-vessels 

 were mentioned. These join up and take blood to the heart, 

 which is a very different organ from ours. It is simply a 

 slightly swollen tube along which waves of contraction pass, 

 thus forcing the blood on its way. After a certain number of 

 pulsations, about 70, in one direction they cease and recom- 

 mence in the opposite direction. Hence the blood-vessels 

 become veins and arteries alternately. 



The nerves that govern the actions of the animal proceed 

 from a solitary ganglion which is situated between the two 

 apertures. Sense organs are absent, but sensory cells are 

 found in various parts of the body, notably round the orifices. 

 The thin margins of the siphons are apparently the most 

 sensitive regions. 



Ascidians are hermaphrodite and the egg gives rise to a 

 free-swimming larva. Great interest centres around this 

 larva, because it tells us a wonderful story, for it proves that 

 the Ascidian parent, although seemingly a mere lifeless, 

 motionless lump of jelly, is in reality a vertebrate in disguise. 

 The larva, which closely resembles a tadpole, swims by means 

 of a long tail ; it possesses a notochord supporting a spinal 

 cord swelling anteriorly into a brain ; it has a single eye with 

 retina and lens, besides an organ of hearing. 



Its swims actively, but for a very short time, usually 

 much less than a day, and then settles down head first and 

 attaches itself by means of cement organs on the head. Then 

 commences the retrograde metamorphosis leading to the full 

 grown stage. The tail is drawn in, the notochord and spinal 

 cord are absorbed or dissolved in the body juices, the brain 

 disappears and the nerve tissue dwindles down to the single 



