474 TUNICATES 



in Professor Herdman's Memoir on Ascidia (" Liverpool Marine 

 Biology Committee Memoirs," No. I., 1899). We do not pro- 

 pose to do more than give a general picture of the structure, 

 enough to serve as a basis for some remarks on the natural 

 history of these animals. 



A common shape is like a double-necked leather bottle 

 (Plate XXXIV., D) and it is to this that the word Ascidian 

 refers. At the upper end there is an inhalant opening or 

 mouth ; somewhat to the side there is an exhalant aperture. 

 When the Ascidian is touched, it can send a jet of water from 

 either opening, hence another name, " sea-squirt ". The name 

 Tunicate refers to the clear " test " or " tunic " which sur- 

 rounds the animal and fixes it to rock or sea-weed. This 

 " test " begins as a structureless cuticle, — a non-living layer 

 secreted by the underlying living skin, and it has the peculiar 

 interest of consisting mainly of cellulose, which is a very 

 characteristic vegetable substance, forming the cell-walls in 

 plants. In a very passive part of a very sluggish animal we 

 find a characteristic vegetable substance. As the animal grows 

 a little older, migrant cells (from the middle embryonic layer 

 or mesoderm) wander into the cellulose test, and some cells of 

 the epidermis seem also to pass in, so that what began as a 

 cuticle ends with being a sort of tissue, including pigment cells, 

 bladder-like cells, and outgrowths of the body-wall with blood- 

 channels. 



When the test is cut away, the body-wall of the Ascidian 

 is seen, and there are two points of special interest. It is tra- 

 versed by long spindle-shaped unstriped muscle-fibres — the 

 kind of muscle-fibre that is found in sluggish animals or in the 

 slowly-moving internal parts of active animals, such as the wall 

 of the food-canal and the bladder. Between the two openings, 

 lying on the margin, there is an elongated nerve-ganglion, the 

 remains of the anterior part of the dorsal nerve-cord which is 

 present in the larva. 



This ganglion defines the dorsal median line, and if we now 

 hold the animal in a position similar to that of our own body, 

 we see that it is the right-hand side of the animal that is so 

 abundantly traversed by the muscle-fibres, while the left-hand 

 side is quite different, showing a twisted intestine and reproduc- 

 tive organs. 



