216 TORREY 



part. A very constant muscle-cell is one that runs from the 

 oesophagus to the stomach on the dorsal side. Muscles also 

 run from the body-wall to the alimentary tract along its entire 

 course. These muscles are, for the most part, mere threads 

 with a central swollen nucleated part, but the latter seems some- 

 times to be entirely absent. As a rule the contractile thread- 

 like part branches extensively and at both ends. I could not 

 find " the broad loose bands of muscles connecting the apical 

 plate with the lateral ciliated band" figured and described by 

 Conn. It is probable that these were confused with the ventro- 

 cesophageal bands. Besides these muscles there are a great 

 many of what are apparently undifferentiated mesenchyme-cells, 

 which, as Conn has noted, form an almost continuous layer on 

 the inner side of the body-wall. This is also the case in Echi- 

 urus, the trochophore of which is in many respects closely sim- 

 ilar to that of TJialassenia. 



e. Excretory Cells. — A careful search in living and preserved 

 material has confirmed Conn's assertion that there are no pri- 

 mary nephridia in Thalassema. The excretory function seems 

 to be relegated to a number of globular mesenchymatous cells, 

 which are either loosely attached to the body-wall and muscles 

 or float freely in the primary body-cavity (see Text-Fig. 8, A). 

 These are especially prominent in the three- or four-day trocho- 

 phore. They soon become stained a yellowish-brown color 

 and in the oldest trochophores contain refractive granules. In 

 several gastropod larvae excretory cells have been described, 

 and they are in every case of ectodermal origin. They are often 

 found closely associated with the velum, e.g., Bythinia (Sarasin, 

 '82), Ochidiiun (Joyaux-Lafifine, '82), Neritinia (Blochmann, 

 '82), Crepidtda (Conklin, '97). Of greater interest in this con- 

 nection, however, is the fact that in certain adult tunicates, e.g., 

 Botryllus, Polycycliis, Ciona and Salpa, the excretory organ, ac- 

 cording to Dahlgrun ('01), is represented by a small number of 

 unmodified mesenchyme-cells in the protoplasm of which the 

 excretory products form dark granules. 



f. Habits. — The trochophore swims in the usual way with 

 the apical plate directed forward and revolving spirally on its 



