4 8o TUNICATES 



the Mediterranean, which attains a length of 3 centimetres and 

 has a bright red colour that is not uncommon in abyssal 

 animals. 



The " house " of Appendicularians is a very interesting 

 structure, illustrative of the multiple utility that is often found 

 justifying the existence of what seem at first sight like exu- 

 berances. It is a gelatinous non-living cuticular investment, 

 made by tracts of large ectoderm cells, and quite transient. 

 Lohmann has shown that it has some hydrostatic value, that 

 it is protective since the tenant can escape its enemies by flit- 

 ting from its house, and thirdly that it serves with its compli- 

 cated passages and pores as a great sieve for microscopic food- 

 particles. 



There are two views as to the position of Larvacea. Ac- 

 cordingly to some they represent the primitive type, and the 

 sedentary Ascidians are more remote. According to others, 

 e.g., Perrier, the sedentary simple Ascidians, e.g., Julin's Medi- 

 terranean Archiascidia, are nearer the ancestral vertebrate 

 stock, and the Appendicularians are specialised pelagic forms 

 which remain at a larval level. 



Order II. Ascidiacea or Ascidians 



In contrast to the Larvacea, the Ascidians have in adult 

 life no trace of tail or notochord ; they have a permanent test 

 into which cells migrate ; they have a large respiratory pharynx 

 and the primary gill-clefts open to the exterior are replaced by 

 usually numerous secondary slits opening into a peribranchial 

 or atrial cavity. There are three groups, the sedentary Simple 

 Ascidians, the sedentary Compound Ascidians, and the free- 

 swimming colonial Pyrosomes. 



The Simple Ascidians are usually solitary, but they are 

 linked to the Compound Ascidians by the family Clavelinidae. 

 These Clavelinids are, on the one hand, the simplest of Simple 

 Ascidians, and, on the other hand, by forming small groups of 

 connected individuals, they reach on to the Compound Asci- 

 dians. A single individual buds off others, and these bud off 

 others, till a small "gemmarium" is formed, in which each 

 member has a distinct test, and is complete in itself, yet all 

 share the same blood -system, and are connected by (epicardiac) 



