696 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxxv. 



entire colony, their disposition indicating that they serve to power- 

 fully contract and shorten the rachis. The bands are immersed in 

 connective tissue which is more or less vesicular, containing numer- 

 ous irregular lacunae which may be a part of the water-vascular sys- 

 tem. (Plate XCI, fig. 1.) 



Intervening between the muscle bands and the lining of the rachis 

 cavity is a mass of spongy tissue and a rather ill-defined layer of 

 circular muscle bands. 



To recapitulate. Passing from the surface of the rachis inward we 

 encounter the following structures in regular order: 



1. A superficial layer of zooids in longitudinal section, with the 

 whole layer thrown into deep convolutions in places, and numerous 

 spicules embedded in the upper parts of their walls. 



2. A narrow 7 zone of canals leading downward from the body cavi- 

 ties of the zooids. 



3. A zone occupied by the cross sections of regularly arranged longi- 

 tudinal canals, separated by partitions of connective tissue. 



4. A very conspicuous zone of sections of powerful longitudinal 

 muscles embedded in connective tissue and with numerous lacunae. 



5. A zone of loose, spongy tissue, doubtless part of the erectile 

 tissue of the rachis. 



6. An inconspicuous layer of circular muscles. 



7. The lining of the cavity of the rachis. 



A section taken parallel to the surface of the mass of zooids, but 

 deep enough to include the upper part of the oesophageal tubes, 

 shows very plainly the wall of the body cavity, the eight mesenteries 

 in section, the endodermal lining of the oesophageal tubes and a trans- 

 verse section of the conspicuous siphonoglyphs with a very unusual 

 display of strong, lash-like cilia. For details of this interesting sec- 

 ion see fig. 2, P*ate XCI. 



Below the oesophageal tubes the mesenteries are mucft reduced, 

 and sometimes entirely w r anting. Often one or two can be made out, 

 but this arrangement does not seem to be constant. 



THE CIRCULATORY SYSTEM. 



The different parts of the w T ater system have been mentioned in 

 the above account, but not in such a manner as to show their ana- 

 tomical relations as a whole. 



It is possible for water to enter or leave the colony either through 

 the opening at the distal end of the stem, the mouths of the polyps, 

 <>i- the mouths of the zooids. It seems likely that the extreme disten- 

 sion of the erectile tissue of the stem is effected by water entering the 

 stem cavity by means of the opening at the end of the stem, and 

 passing into the spongy erectile tissue of the stem walls by means of 

 1 he "stomata" shown in fig. 3, Plate LXXXV. Probably this is the 



