200 



ASCIDIANS OF GUERNSEY. 



large and usually brilliantly coloured. In fact the colour of 

 these animals varies between black and white in Botryllus 

 morio to brilliant vermilion in Leptoclinum lacazii. The free- 

 swimming forms mentioned above are phosphorescent, as are 

 also many of the deep-water species. 



One of the large Simple Ascidians may be described as 

 an example of the class. The structure of the individuals in 

 the colonial species is essentially the same as that of a simple 

 one. Let us take the common Ascidia mentula. 



This is a large form, often five to six inches long, of a red 

 colour and lj.ithery to tha touch. This leathery feel is due to 

 the thick outer covering, called the test, which is the pro- 

 tecting layer of the body. In our example it is about a quarter 

 of an inch thick, an I s nail molluscs and crustaceans inhabit 

 holes in its substance. In some species, however, the test is 

 soft and gelatinous. Lining the test is a delicate membrane 

 by which it is secreted. 



On pressing the animal two streams of water are ejected 

 with some force. One comes from an opening at or near the 

 apex, called the Branchial Orifice ; while the second stream 

 of water is emitted from another opening further down one 

 side known as the Atrial Orifice. 



On close inspection the sides of both openings are found 

 to be split into lobes — in our example eight for the Branchial 

 and six for the Atrial. This is an important character be- 

 cause the number of lobes is exact and constant for certain 

 genera and families, e.g., Molgula, 6 and 4 ; Cynthia, both 

 4 lobed ; Ascidia, 8 and 6 ; Diazona, both 6 lobed ; while 

 those of Clavellina are not lobed at all. 



The branchial aperture is anterior, that is to say, it 

 corresponds with the head region of man, while the atrial 

 aperture is dorsal, corresponding to the back region of man. 



The apical opening or branchial orifice leads into a 

 wonderful sac suspended in the cavity of the Ascidian. This 

 sac, which is the pharynx, is called the branchial sac and its 

 walls are pierced by innumerable slits called " stigmata." Its 

 substance is hollowed out by countless blood vessels and the 

 water continually washing through the stigmata oxygenates 

 the blood. Hence the branchial sac is the breathing organ 

 of the Ascidian. The water, after passing through the stig- 

 mata, goes into the general body cavity or atrium and escapes 

 through the atrial aperture. 



The branchial sac gradually narrows posteriorly and 

 finally leads into the oesophagus and thence to the stomach. 

 The mechanism required to transfer food to the stomach is 



