388 L. DONCASTEB. 



their body-cavities has been considered as a reason for 

 associating thera with Balanoglossus, and possibl}' with the 

 Echinoderms and Phoronis. I will discuss first their resem- 

 blances with the Annelida. 



The resemblances between the Chaetognatha and the 

 Annelida were pointed out by Huxley half a century ago, and 

 have since been emphasised by Hertwig and others. In 

 transverse section there is a considerable likeness between 

 Sagitta and an Annelid, — such, for example, as Polygordius. 

 In each there is a large body-cavity divided by a tongitudiual 

 mesentery, which supports the alimentary canal ; in each 

 there are four groups of longitudinal muscles, which appear 

 pinnate in section, and in which the fibres are striped. There 

 are no circular muscles, and the epidermis is simple. If the 

 section be taken through the ventral ganglion, the latter 

 corresponds in position with the nerve-cord of Polygordius, 

 and from its greater size in the larva may perhaps be 

 regarded as a longitudinal cord which has become aggregated 

 into a single ganglion. The section of Polygordius differs 

 from that of Sagitta in the possession of oblique septa, 

 nephridia, and blood-vessels. 



Further resemblances with the Annelids are found in the 

 cerebral ganglion connected with the ventral by circum- 

 ocsophageal commissures, and in the transverse segmentation. 

 It is supposed by the advocates of the Annelid theory that 

 Sagitta is an animal of three somites, separated from one 

 another by two transverse septa, one in the neck and the 

 other near the anus. The number of segments has become 

 greatly reduced, but those that remain are closely similar to 

 the typical annelid somite, except that the alimeutai'y canal 

 is not continued into the last one j since, however, embryology 

 shows that the endoderm is continued to the tail, this objec- 

 tion has not much Aveight. Another important difference 

 is the absence of nephridia in Sagitta, and it has been 

 supposed that they are represented in a modified form by the 

 genital ducts. It lias been further argued that the hooks of 



