RHABDOPLEURA. 



113 



canal (./). The collar-cavities are continued into the arms, which 

 arise from the collar region, and the trunk cavities appear to be 

 continued into the stalk. At any rate the stalk contains in addi- 

 tion to its muscular tissue two cavities separated by a septum.* 

 The proboscis-cavity possesses a pair of pores, and the collar- 

 cavities communicate with the 

 exterior, each by a collar canal. 

 On the dorsal side of the collar 

 region behind the collar coelom 

 (Fig. 82), there is an ectodermal 

 thickening which seems to corre- 

 spond with the nerve plate of 

 Cephalodiscus and the medullary 

 nerve cord of the Balanoglossida. 



Mesoblastic skeletal tissue, ap- 

 parently similar in character and 

 relations to that of the Balano- 

 glossida, occurs in the arms and 

 tentacles, and in the axis of the 

 stalk. 



Most of the specimens hitherto 

 found have been devoid of 

 generative organs, but in a few 

 there was a testis in the form of 

 an elongated sac lying parallel 

 to the intestine on the right side, 

 and forming a projection on the 

 surface of the body ; it opened 

 close behind the anus. 



A vascular system consisting 

 of a dorsal and ventral vessel 

 has been described, and there appears to be a pericardial vesicle 

 and heart at the anterior end of the notochord as in other 

 Enteropneusta. 



Rhabdo pleura^ is a marine animal and is found in compara- 



c Fowler thinks that the stalk contains a cord of endoderm which is 

 continuous with that of the alimentary canal. If this is so, the endoderm 

 may participate in the budding. 



t It has been suggested that the section of the Graptolites known as 

 Monograptidae are nearly related to Rhabdopleura (see Alhnan, On the 

 Morphology and Affinities of Graptolites, Ann. and Mag. Nat. Hist., 

 1872. and Schepotieff, Neues Jahrbuch /. Mineralogie, 2, 1905, pp. 79-98). 



Z III I 



FIG. 82. Rhabdopleura normani in longi- 

 tudinal vertical section (after Fowler 

 from Lang). 1 arm of one side, indi- 

 cated by dotted lines ; 2 anterior paired 

 (collar) coelom ; 3 anus ; 4 posterior paired 

 (trunk) coelom ; 5, 6 alimentary canal ; 

 7 buccal cavity ; 8 mouth ; 9 anterior 

 diverticulum of buccal cavity(notochord); 



10 anterior impaired (proboscis) coelom ; 



11 buccal shield. 



