580 THE ANATOMY OF INVERTEBRATED ANIMALS. 



though it can hardly be said to fill up the hiatus between 

 them. 



In our further search after the serial relations of animals, 

 we must therefore start afresh from the lowest Metazoa. 

 Here a ZOOPHYTIC SERIES is very well marked ; commencing 

 with the Physemaria, and thence diverging, on the one 

 hand, to the Porifera, and, on the other, to the Coelenterata, 

 with the highest forms of which this series comes to an end. 



A second gradation, which may be termed the ANNTJLOID 

 SERIES, is represented by the Trichoscolices and the Anne- 

 lida. The lowest Turbettaria are upon nearly the same level 

 of organization as the Hydrozoa. It would be hard to dis- 

 tinguish an aproctous Turbellarian, devoid of a ganglion and 

 water-vessels, from a free-swimming nontentaculate Hydro- 

 zoon. On the other hand, as I have already pointed out, the 

 line of demarcation between the higher Trichoscolices and 

 the Annelida is very indistinct, and we may expect it to be 

 speedily obliterated by the progress of discovery. 



A third gradation is constituted by the Nematoscolices and 

 the Arthropoda. The lowest Nematoidea possess no higher 

 organization than the lowest Turbettaria and the Motifera. 



The NematorhyncJia, whether they are really transitional 

 forms between the JVematoidea and the Arthropoda or not, 

 at any rate indicate the road by which the transition may be 

 effected ; and I am much inclined to think that the Chceto- 

 gnatha may occupy a place in this series. The oral armature 

 of Sagitta may be regarded as a modification of the oral 

 spines of Echinoderes, and its nervous system is as much 

 Arthropodal as is that of the Pentastomida. This may be 

 called the ARTHROZOIC SERIES. 



A fourth series is that which I shall term the MALACOZOIC 

 SERIES. It includes the Malacoscolices and the Mollusca. 

 The entoproctous Polyzoa form the lowest term of this series. 

 The resemblances of the Polyzoa with the JRotifera (e. g., 

 with Stephanoceros) have often been remarked, and, indeed, 

 insisted upon, with too little regard to the differences which 

 are established by the water-vessels and the peculiar pharyn- 

 geal armature of the Rotifers. Nevertheless, these resem- 

 blances are important as far as they <ro, and in arade of or- 

 ganization the two groups are much upon the same level. On 



