TEE SEA SQUIRTS. 



253 



CYNTHIA (ASCIDIA) MICKOCOSMUS. 



they are met with as gregarious assemblies of individuals, and at others as solitary examples. They 

 are oviparous, and the sexes are united. 



On the coasts of the Channel, the Mediterranean, and in the China seas and in Brazil, some of 

 the species of these simple Ascidians are valued as articles of food. "At Cette, Ascidia are taken 

 regularly to market, and Cynthia microcosmus, although so repulsive externally, furnishes a very 

 delicate morsel much sought after." (Van Beneden.) The young Ascidians commence life as free- 

 swimming, tadpole-like embryos. The tadpole as it appears in the egg is at first an oval disc ; a tail 

 is soon after observed ; arm-like projections spring from the head of the creature, which then presents 

 a striking analogy with the form of a hydroid Zoophyte ; it becomes free, and swims about by means 

 of its rapidly vibrating tail ; it fixes 

 itself to rocks and seaweeds by its 

 arms ; the tail disappears ; that which 

 was the head, or nucleus, sends out 

 root-like projections, orifices appear 

 in it, and its final form as an Ascidian 

 begins to be manifested. 



The characters of the genus 

 Ascidium* are body sessile, covered 

 with a leathery or gelatinous tunic ; 

 branchial orifice eight-lobed; atrial 

 orifice six-lobed ; branchial sac not 

 plicated, having a circle inside of 

 simple tentacular filaments ; the 

 meshes of respiratory sac papillated. 



The leathery sac is exceedingly 

 muscular and contractile, and from 



the rapidity with which (when touched) they eject the water contained in their bodies, they are 

 popularly known as "Sea-squirts." This outer covering is very remarkable as containing a consider- 

 able proportion of a substance apparently identical with cellulose, which is one of the most 

 chai-acteristic of all vegetable products. 



The Ascidia are found attached to the under side of rough stones, and vary in length from one to 

 six inches. They are variously, often splendidly, coloured, but are otherwise unatti-active. Numbers 

 of them are often found clustering among tangles, like bunches of some strange semi-transparent 

 fruit. (E. Forbes.) 



They range from low water to twenty fathoms, attached to rocks, shells, and the like, twenty species 

 being found in Britain, the Mediterranean, Greenland, Spitzbergen, the United States, and elsewhere. 



Genus Molgida.\ The body is attached or free, and more or less globular in form; the orifices 

 are very contractile naked tubes ; the oral is six-lobed, the atrial four-lobed. They have been met 

 with between tide-marks in the laminarian zone, and down to a depth of twenty-five fathoms. 

 The surface is membranous, and is usually covered with particles of sand and other extraneous 

 substances. Five species ai-e recorded by Adams, from Britain, Denmark, &c. 



Genus Cynthia. The body is covered with a coriaceous tunic, not stalked ; the oral and atrial 

 orifices are each four-lobed ; the branchial sac is longtitudinally plicated. The meshes in the respira- 

 tory tissue have no papilla ; there are two ovaries. The species ranges from low water to thirty fathoms. 

 They are frequently found associated in groups of numerous individuals, 'and their tests, even in the 

 same species, are often variously coloured. They are found on the coast of Greenland, Norway, 

 Britain, and the Mediterranean. They are often gregarious, forming large bunches by the interlacing 

 of their root fibres. 



Genus Pttonaia.% The test is cylindrical ; the body elongated, smooth or wrinkled ; the apertures 



are on two small conical eminences, lower end provided with fine rootlets ; there are two ovaries. Two 



species occur in Britain and Norway. Pelonaia resembles Sipunculus, one of the worm-like Echino- 



derms, in appearance ; it is not free, but rooted in the mud, and quite as apathetic as other Ascidians, 



* Greek, askos, a skin bottle. f Greek, molffos, a bag of skin. J Greek, pelos, mud ; naio, to inhabit 



