566 RECORD OF CURRENT RESEARCHES RELATING TO 



it exhibits many points in adult structure, which are merely embryonic 

 in other forms ; thus segmentation never affects any other than the 

 mesodermal structures, the central nervous system is never placed 

 otherwise than just below the ectoderm, the supra-oesophageal ganglion 

 retains its early (anterior) position, and the ventral cord does not 

 develop ganglionic enlargements. Connecting this form with the 

 higher Cha3topoda is the interesting Saccocirrus, which may be taken 

 as the type of the Archi-chfetopodes, just as Polygordius may represent 

 the Archi-aunelides ; regarding the Hirudinea and Gephyrea as be- 

 longing to the same series, we get an arrangement of this kind : — 



Annelidcs. 



1st order. Polygordiida:. 



I 2ad order. Chretopoda. 



1st suborder. Saccocirridm. 

 I 2nd „ Polychceta. 



4th order. Gephyrea. 3rd „ Oligochaita. 



3rd order. Hirudinea. 



The head of the annelid is regarded, in opposition to Professor 

 Semper, as consisting of a single segment, which is characterized by 

 the possession of an oesophagus and of a supra-oesophageal ganglion, 

 and in having a cavity derived from the primitive coelum, while there 

 are never, as there are ideally in all the rest, generative organs. 

 Whether the annelid is a colony of segments with an anterior sterile 

 individual (the head), or whether it consists of a head and trunk, 

 which latter undergoes a process of gemmation, must still remain a 

 moot question ; but it is not, in any case, to be compared with the 

 Cestoid Worms, which differ altogether in having the youngest segments 

 nearest the head. 



The name troehophore is proposed for the larva of Polygordius. This 

 is, from any point of view, a very instructive form. Found adult in the 

 Eotatoria, it is continually met with in the developmental history 

 both of the Annelides and of the Mollusca ; its oral circlet of cilia 

 consists of a double-rowed pre-oral portion, a single-rowed post-oral 

 circlet, and a ciliated groove separating the two. The frontal nervous 

 plate, which is the rudiment of the supra-oesophageal ganglion, appears 

 very early ; it has been recognized in the Eotatoria and the Mollusca, 

 and the primary tentacles which become connected with it appear to 

 be homologous with the " tentacles " of the last-named group. Diffi- 

 culties presented by the characters of the so-called segmental organs 

 are dealt with in a very ingenious manner ; while as to the generative 

 organs, it is nsggested that in the Eotatoria they have disappeared on 

 one side. Having in mind all these similarities. Dr. Hatschek proceeds 

 to suggest the existence of a common ancestor for the Eotatoria, Anne- 

 lides, and Mollusca, for which he proposes the name of Trochozoon. 



The Bilateria are those animals in which there are three distinct 

 layers, and in which the genital products are derived from the meso- 

 derm. To deal with the different groups : — 



Echinodermata. — The larvae of these forms have no excretory organs 

 and no frontal plate, while the vaso-peritoneal organs are formed from 

 the endoderm, and the adult is of a radiate form. As these animals 



