ORIGIN OF CHORDATES 425 



their allies from the other great group of invertebrates com- 

 prising the Annelida, Arthropoda, and Mollusca. 



There is reason to believe that the concentration of 

 nerve-cells to form a central nervous system out of the more 

 primitive diffuse nerve-net took place in the region of greatest 

 stimulation. This is the ventral side in Annelida, Arthro- 

 poda, and Mollusca, all of which typically crawl on the 

 ventral surface. The fact that the central nervous system of 

 chordates is dorsal seems to show that the ancestral chordates 

 were not ventral crawlers, but pursued a free-living pelagic 

 existence, receiving the greatest stimulation on the dorsal 

 side from the surface of the sea. 



Returning now from these more or less distant allies to 

 true chordates, the next group to consider is one which, like 

 the Hemichordates, has left the main line of chordate evolu- 

 tion and become specialised in different directions : the 

 Urochordata. These preserve the notochord in the tail in 

 the larval stage only, and the dorsal tubular nerve-cord of the 

 larva degenerates in the adult. They possess gill-slits, and a 

 typical well-developed endostyle, used in connexion with the 

 ciliary method of feeding. Their development is also typical 

 of chordates. One group of these animals, the Larvacea, 

 retain the larval structure throughout life, with the tail and 

 notochord. The others pass through a free-swimming larval 

 stage, and then undergo a retrograde metamorphosis into 

 sessile animals, losing the tail, notochord, and larval eyes and 

 organs of balance. These are the Ascidiacea or sea-squirts. 

 Some of these are solitary, but most are colonial, reproducing 

 extensively by asexual reproduction or budding, as is commonly 

 the case with sessile forms. Others, forming the group of 

 Thaliacea or salps, have returned to a free-swimming existence, 

 but retaining many traces of former sessile habits ; in particular 

 the habit of budding, which is very prevalent. Some of them 

 have a true alternation of sexually produced (from fertilised 

 eggs) and asexually produced (from buds) generations, and one 

 form is further interesting in that the sexually produced genera- 

 tion is nourished during its development by the mother by 

 means of a placenta (Salpa). 



