406 



COMPARATIVE ANATOMY. 



fact we have a special divergence from the arrangement obtaining 

 in all other invertebrate animals, for in them the central organ of 

 the circulation is invariably a specialisation of a part of the dorsal 

 vascular stem. Nevertheless there can be no mistake about the 

 connection with that of the Worms in the disposition of the whole 

 apparatus. 



Peculiar to all Tunicates is the alternating direction of the 

 blood-stream set in motion by the heart. In consequence 

 of this change of direction, it is not possible in them to speak of an 

 arterial and a venous division of the vascular system. When the 

 heart has completed a certain number of pulsations in one direction, 



suddenly a moment of quiescence occurs, and 

 then the peristaltic movements of the cardiac 

 tube begin again, but in the opposite direction. 

 This condition of indifference is a further obstacle 

 to the close assimilation of the vascular system 

 »' of the Tunicata to that of any one of the other 

 great divisions ; at the same time, it calls to mind 

 a similar phamonienon of the reversal of the blood- 

 stream in the Gephyraaa (Phoronis). 



'. >i - 



■Si I? 1 '':)'*' 



J " ;■ 



^ 



Fig. 213. Organisa- 

 tion of an Ascidian 

 (Amarcecium pro- 

 Kferum). sb Bran- 

 chial sac. v Stomach. 

 i Intestine, c Heart. 

 t Testis, vd Efferent 

 duct of the testis, 

 o Ovary, d Eggs in 

 the body - cavity. 

 The arrows indicate 

 the direction of the 

 stream of water at 

 the orifices of the 

 body (after Milne- 

 Edwards). 



Organs of excretion have as yet been 

 recognised in the Tunicata to a limited extent. 

 In many Ascidians (Molgula, A. conchilega, com- 

 planata) a tubular organ is found in the neigh- 

 bourhood of the branchial chamber, or placed 

 farther back in the body, which exhibits, besides 

 other cells, some containing concretions. In one 

 species the murexide reaction has been obtained. 

 No openings have been discovered in the orgau, 

 so that the arrangement appears to represent 

 that condition in which excretionary matters arc 

 separated in the organism, and form concretions 

 which are not removed to the exterior. 



Sexual Organs. 

 § oil. 



Only one division of the Tunicata is provided 

 with sexual organs universally — the Copelata. 

 In the others, in consequence of the elaborate 

 asexual reproduction, a large proportion of indi* 

 viduals are devoid of sexual organs ; the absence 

 of which is explained by the production of germs, 

 a modification of the process of multiplication by 

 budding (cf. p. 391). 

 The hermaphrodite arrangement, common among Tunicata, is 

 often found at a very low stage of development. The Appen- 



