378 EMBRYOLOGY 



nature of a true ca'lom and to be filled with connective 

 tissue and muscles (Köhler). 



The two chief vascular trunks of Balanoglossus (Figs. 1G5 

 and 166 vb and db) extend in the ventral and dorsal middle 

 lines between the wall of the intestine and that of the body. 

 The blood flows forward in the dorsal vessel, backward in 

 the ventral. They give off branches at regular intervals, 

 which extend to the body-wall, to the intestine, and to other 

 organs. According to Kowalevsky, there are also two 

 lateral trunks which receive vessels from the intestine and 

 from the gills. Their presence was confirmed by Köhler. 

 It still appears doubtful whether the saccular structure lying 

 at the base of the acorn and at least connected with the 

 vessels of the body, which was maintained by Köhler and 

 Bateson to be the central organ of the blood-vascular system, 

 is to be looked upon as a heart. In Fig. 165 it (h) is seen 

 lying on the dorsal side of the intestinal diverticulum. An 

 organ (x) lies above it the significance of which is still less 

 certain. It is a closed saccular body, the epithelial lining of 

 which is greatly thickened anteriorly (Fig. 165 x). Owing 

 to its intimate relation to the blood-vascular system, due to 

 its position, Spengel looked upon the anterior part of this 

 organ as an internal gill (acorn gill), whereas Batesox and 

 Köhler explain it as a gland (proboscis gland), which has 

 an excretory function. To be sure, a difficulty occurs with 

 this explanation, namely, the absence of the efferent duct ; 

 for it is not evident how the proboscis pore conveys away the 

 products of this " gland," which is a closed sac. Apart from 

 this, and in the absence of any other excretory organ, this 

 interpretation is nevertheless natural. 



A thick cord, which lies in the dor.sal mid-line of the collar, 

 is to be looked upon as the central organ of the nervous 

 system (Fig. 165 dn). It is said to possess a cavity which 

 would be compai'able to the central canal of the spinal cord 

 of the Vertebrata (Bateson), but this is denied by Spengel. 

 According to both Spengel and Köhler, the cavity is 

 traversed by cords of cells, so that only irregular spaces 

 appear in it. Köhler further states that the following pecu- 

 liar condition exists : the internal space of the nerve cord 



