580 THE ANATOMY OF INVERTEBRATED ANIMALS. 



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

 them. 



In our farther search after the serial relations of animals, 

 ■we must therefore start afresh from the lowest 3fetazoa. 

 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^ 

 licla. The lowest Turhellaria 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 Hj^dro- 

 zoon. On the other hand, as I have alread}^ pointed out, the 

 line of demarcation between the higher TrichoscoUces 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 N'ematoscolices and 

 the Arthropoda, The lowest Nematoidea possess no higher 

 organization than the lowest Turhellaria and the Rotifer a. 



The JSfematorhyncha, whether they are really transitional 

 forms between the Nematoidea 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 occup}'^ 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 Pentastoniida, This may be 

 called the Arthrozoic Series. 



A fourth series is that w^hich 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 Potifera (e. g., 

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

 insisted upon, with too little regard to the diff*erences which 

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

 geal armature of the Rotifers. Nevertheless, these resem- 

 blances are important as far as they go, and in grade of or- 

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



