, 43 



by processes of the test, each enclosing a diverticulum 

 from the body -wall, so as to form " chains." Each 

 member of the chain is a Salpa of the sexual or aggre- 

 gated form, and when mature may — either still attached 

 to its neighbours or separated from them — produce one 

 or several embryos, which develop into the solitary Salpa. 

 Thus the two forms alternate regularly. 



Salpa, like Doliolum, is probably only an occasional 

 visitor in our seas, but several species of the genus — 

 Salpa democratica-mucronata, S. ruucuiata-fusiformis, 

 and S. zo)iaria^\SbYe been found on occasions in the seas 

 of the Hebrides, or cast ashore on our southern and 

 western coasts. 



Order III. ASCIDIACEA. 



Fixed or free-swimming Simple or Compound Ascidians, 

 which, in the adult, are never provided with a tail, and 

 have no trace of a notochord. The free-swimming forms 

 are colonies, the Simple Ascidians being always fixed. 

 The test is permanent and well developed ; as a rule, it 

 increases with the age of the individual. The branchial 

 sac is large and well developed. Its walls are perforated by 

 numerous slits (stigmata) opening into the peribranchial 

 cavity, which communicates with the exterior by the atrial 

 aperture. Many of the forms reproduce by gemmation, 

 and in most of them the sexually produced embryo develops 

 into a tailed larva. 



The Ascidiacea includes three groups — the Simple 

 Ascidians, the Compound Ascidians, and the free-swim- 

 ming colonial Pijrosoma. 



Sub-order I. Ascidi^ Simplices. 



Fixed Ascidians which are solitary and very rarely 

 reproduce by gemmation ; if colonies are formed, the 



