170 



THE PINEAL ORGAN 



brates have been evolved. One important distinguishing character of the 

 Hemichorda may be noted, which serves to connect them with the verte- 

 brate stock, namely, the presence of a series of paired gill slits behind the 

 " collar region," which communicate with the pharyngeal region of the 

 alimentary canal (Fig. 123, p. 169). Somewhat similar gill slits are also 

 present in larval tunicates, which combined with a ventral position of 

 the heart and other definitely vertebral relations of the intestinal organs 

 indicate that these lowly organized animals, the Protochordata, are side- 

 branches of the early vertebrate stem ; but it must not be concluded that 

 they are in the direct line of descent, or, in other words, belong to the true 



cilb 



oc p 



Fig. 124. — The Apical Plate and Eyes of a Full-grown Tornaria Larva 

 of Balanoglossus. (After Morgan.) 



A. Apical view showing the relation of the plate to the longitudinal ciliated 



band. 



B. Antero-posterior, median section through the apical plate and eyes. 



/. cil. b. : longitudinal ciliated band. 



m.c : muscle cells belonging to the apical cord. 



oc. a. : anterior eye. 



oc. p. : posterior eye. 



parental stock of the vertebrates . The intermediate position of Balanoglossus 

 is indicated by the co-existence in it of both dorsal and ventral nerve cords, 

 as well as the larval characters already mentioned which are common to 

 both vertebrates and invertebrates, and, as we shall see later, there are 

 many points which indicate that some of the best known living repre- 

 sentatives of the Urochorda and Euchorda, such as the Ascidia and 

 Amphioxus, have descended from animals which were more highly organized 

 than their living representatives. This conclusion is partly based on 

 general considerations and partly on the differences to be observed in the 



