jo6 DESCRIPTIONS OF PREPARATIONS. 



branches also (= connectives of Hancock) from the pharynx to the body walls, 

 thence to the test and back again. When the heart pulsates so as to drive the 

 blood from the ventral to the dorsal surface, it draws arterialised blood from 

 the pharynx and venous only from the test ; when in the opposite direction the 

 blood it draws has previously passed through the viscera, body walls, and test. 

 The blood itself is a clear plasma containing rounded nucleated corpuscles, 

 many of which are pigmented, generally yellow, red, or brown, but white and 

 blue are sometimes found. 



All Urochorda are hermaphrodite, but in most instances the male and female 

 organs are not ripe at the same time. These organs in Ascidia are racemose 

 glands placed on the left side of the body between the intestine and stomach. 

 The testis is composed of delicate white tubules, ramifying dichotomously, and 

 spread over the ovary, stomach, and intestine. Both oviduct and vas deferens 

 run along the dorsal edge of the intestine and open near the anus. 



The Ascidian is an example of an animal which has lost, more or less com- 

 pletely, the structure typical of the phylum, and even, strictly speaking, of the 

 class to which it belongs, at the same time acquiring marked peculiarities of its 

 own. It is an instance of what has been termed by Professor Ray Lankester 

 Degeneration, or 'a gradual change of structure in which the organism becomes 

 adapted to less varied and less complex conditions of life.' Elaboration is the 

 converse : but ' elaboration of some one organ may be a necessary accompaniment 

 of Degeneration in all the others : in fact this is very generally the case.' It is 

 when the total result of both processes combined leaves the organism ' in a lower 

 condition, that is, fitted to less complex action and reaction in regard to its surround- 

 ings than was the ancestral form with which we are comparing it (either actually or 

 in imagination), that we speak of that animal as an instance of Degeneration.' 



The causes of degenerative evolution are, according to the same authority, 

 four in number : (i) parasitism ; (2) fixity or immobility; (3) vegetative nutrition; 

 (4) excessive reduction of size. Instances are, of the first, various Copepoda, 

 e.g. Lernaea; of the second, a barnacle (Lepas}; of (i) and (2) combined, the 

 parasitic Cirripedia or Rhizocephala ; of the third, the Turbellarian worm Convoluta ; 

 and of the fourth male Rotifers, etc. 



There can be no reasonable doubt that an Ascidian is one of the Chordata. 

 The structure and development of the larva suffice to establish this point fully. 

 The central nervous system originates as a neural groove the side of which closes 

 over to form a neural tube; there is a cerebral or myelonic eye; the caudal notochord 

 is derived from the archenteron ; the pharynx becomes a respiratory organ pierced 

 by slits, with walls richly vascular. These structures reach a certain degree of 

 perfection. But with the fixation of the larva a series of regressive changes sets 

 in. The notochord disappears with the swimming tail. The nervous system is 

 reduced to a fraction of what it was in the larva. The eye is lost. The pharynx 

 however becomes much more complicated and enlarged relatively to the remaining 

 organs. The larval characteristics are however more or less retained and specialised 

 in the free swimming order Larvacea. 



Tunicata. Herdman, Challenger Reports, vi. 1882; and Bronn, Klass. und 

 Ordn. des Thierreichs, Malacozoa, iii. i. 1862. 



Notes on British Tunicata. Herdman, J. L. S. xv. 1881. Ascidies Simples 



