ASCIDIANS AND LANCELETS 199 



brain, cerebral eye, gill-slits, ventral heart, etc. As the adult 

 stage is reached these characteristics disappear, the tail shrivels 

 up, the notochord and spinal cord vanish, the brain is only repre- 

 sented by a small ganglion, and the whole symmetry of the body 

 is altered. 



Professor Milne-Edwards divides the Tunicates into three sub- 

 orders, of which the Ascidia, the Salpa, and the Pyrosoma are the 

 types, and he subdivides the Ascidians proper into simple, social, 

 and compound. In the Simple Ascidians the body is sac-shaped, 

 gelatinous or leathery, fixed at one end and free at the other. It 

 has two more or less prominent orifices, one the oral or mouth- 

 opening, the other the atrial or excurrent aperture. The Simple 

 Ascidians, though at times met with as gregarious assemblies of 

 individuals, are not united into groups by a common integument. 

 In Clavellina we have an example of a social compound Ascidian, 

 i.e. each individual has its own heart, respiratory system, and 

 organs of nutrition, but is fixed on a stalk or base common to the 

 group, through which the blood circulates in opposite directions, 

 like the ebbing and flowing of the sea. In the true Compound 

 Ascidians the separate envelopes are fused, and lose their indivi- 

 duality, forming a common covering in which all the Ascidians are 

 embedded. 



In the family Pyrosomidae 1 the animals are compound and free. 

 The body is cylindrical, hollow, non-contractile, open at one end 

 only, and covered externally by the numerous pointed zooids. 

 The Pyrosomes are from 2 to 14 inches long, and from \ inch to 

 3 inches in circumference, made up of innumerable individuals 

 united side by side. They become most beautifully phosphorescent 

 at night, and a vast shoal of these miniature pillars of fire, gleam- 

 ing out of the dark sea, presents a most wonderful and exquisite 

 sight. The Salpida? are free, alternately solitary, and united in 

 circular or lengthened groups. These Salpa chains vary in length 

 from a few inches to many feet. Chamisso discovered that the 

 solitary Salpae do not belong to species distinct from those united 

 in chains, however dissimilar, but are either the parents or progeny, 



groove running medianly along the back of the embryo. This groove closes to 

 form a tube of nervous matter, the cavity of which always persists throughout life 

 as the central canal of the spinal cord and its anterior prolongation which consti- 

 tutes the ventricles of the brain." Prof. Sidney F. Manner, F.R.S. 

 1 Greek, pyros, fire ; sonta t a body. 



