xviii SUMMARY 661 



other, but later, when some members of this widespread stock took to 

 a creeping life, then the lower cells, in contact with the nutritive 

 substratum, remained nutritive, but the upper cells became protective, 

 and formed a covering dermal layer ; and so the group of Sponges or 

 Porifera were evolved. 



In those members of the stock which remained free-swimming, 

 however, the direction of progression changed from an indefinite 

 rotation to a definitely directed progress ; the hinder cells became 

 adapted to catch nutriment, the anterior became purely sensory. 

 The nutritive cells became increased in number and invaginated, and 

 so the primary gut was formed, and the stock were no longer blastulae 

 but gastrulae. 



The gastrula stage exists as a larval phase in more cases than 

 does the blastula one, and it is likewise represented, in obscured form, 

 in the embryonic history of the higher types. Some of the gastrula 

 stock likewise took to a bottom life, and gave rise to the Hydrozoa, 

 Scyphozoa, and Actinozoa, amongst Coelenterata. 



The main portion of the stock remained, however, free-swimming, 

 and developed lateral pouches of the gut, in which excretory and 

 reproductive functions were specialized, and which also gave rise to 

 cells of a specially locomotor character ; while at the aboral pole the 

 sensitive cells had become a definite nerve centre. 



In this way a primitive wide - ranging group was evolved, 

 supplanting the gastrula stock, to which we may give the term 

 Protocoelomata. In much modified and specialized form this stock 

 survives as the Ctenophora at the present day. But the Proto- 

 coelomate stage in the development of the race is represented in the 

 ontogeny of Invertebrata by three distinct types of larvae, viz. the 

 Trochophore larva, the Echinoderm larva, and the Tornaria larva. The 

 latter two types of larva probably do represent the same type of 

 ancestor, but the Trochophore larva, with its early specialized develop- 

 ment, is different ; it is more closely allied to existing Ctenophora, 

 and the coexistence of these two types points to the existence of 

 different types of specialization amongst the original Protocoelomate 

 stock. The Brachiopod larva is in many ways intermediate in 

 character between the two types. 



From the division of Protocoelomata represented by the Trocho- 

 phore, bottom-living forms were produced. These, as we have pointed 

 out in the proper place, are the burrowing Annelida, the creeping and 

 gliding Mollusca, as well as Podaxonia, and Ectoproct and Entoproct 

 Polyzoa. The Nemertinea are also bottom-haunting forms descended 

 from the same stock. The Platyhelminthes represent an earlier off- 

 shoot from the Trochophore stock, before the primitive mouth had 

 been cut into definite mouth and anus, and the lateral branches of 

 the gut definitely specialized as coelom. The Brachiopoda are a 

 bottom-living group, descended from a type of Protocoelomata inter- 

 mediate between the Ctenophore-like ancestor presented by the 

 Trochophore larva and the Dipleurula ancestor of Echinodermata. 



