OLIGOTREMA PSAMMlTES. 247 



apei'ture^ the lobes of Ctenicella would bear a strong I'e- 

 semblance to the arms of Oligotreina. In tbe former genus 

 the atrial aperture is also surrounded by denticulate lobes, 

 four in number. These are not present in Oligotremn, but 

 their absence is easily accounted for by the rcductioii of the 

 atrial siplion. In my specimen the muscular branched lobes, 

 a single one of wliich is shown in fig. 5, were so much shrunk 

 by the action of spirit that they fitted loosely into the 

 corresponding cavities of the test (fig. 4); but there can be 

 no doubt that in life they fitted close into these cavities^ and 

 that each arm, though covered by a tough coat of test, was 

 capable of a considerable amount of movement. It should 

 be observed that the muscles are not continued into tbe 

 lateral processes wliicli must have fitted into the pinnaSj and 

 it would therefore appear that while the arm could move as a 

 whole, the pinnte were incapable of independent movement. 

 Every muscle-band is made up of a number of fairly long 

 tapering muscle-fibres, cacli with a nucleus situated at about 

 the middle of its length. The individual fibres exhibit a 

 faint longitudinal striation and are cresceutic in cross-sectioUj 

 the nucleus lying in the concavity of the crescent (fig. 29, 

 mus.). I was unable to find any trace of undifferentiated 

 protoplasm surrounding the nucleus. 



The Atrium and Peribrauchial Cavities. 



The relations of the atrium and peribranchial cavities are 

 best seen in the series of sections, figs. 8 — 15, and are also 

 indicated in the diagram fig. 34, which is a reconstruction of 

 my series of sections. The lumen of the atrial siphon is 

 narrow (fig. 15, at. si.) ; internally it opens into the wide atrium 

 or cloaca, at., which lies, like a flattened sac, over the dorsal 

 surface of the stomach, and extends right and left over about 

 one third of its lateral surfaces. As explained above, the 

 test is invaginated for a considei-able distance at the atrial 

 aperture, and forms a thin but perfectly distinct lining to 

 the narrow atrial siphon, extending as far as the openings of 



