THE PHYLUM CHORDATA 



65 



tween the Hemichordata and the vertebrates is through Amphioxus, 

 especially in the close resemblance between the gill-sUts and gill-bars 

 of the two forms. 



Ohder II. Ptekobranchia 



These are forms in which sessile life has profoundly modified the 

 primitive structures. There are two genera, Cephalodiscus and 

 Rhabdopleura. Many species of Cephalodiscus (Figs. 37 and 38) are 

 found in both deep and shallow water, mostly oriental in distribution. 

 Though only about two or three 

 millimeters in length, the individual 

 Cephalodiscus has certain unmis- 

 takable resemblances to Balano- 

 glossus. As an adaptation to sessile 

 life the whole body is bent into a 

 U-shape so that the mouth and anus 

 open close together. Instead of a 

 digging proboscis, it has a flattened 

 structure called the "buccal shield" 

 that overhangs and conceals the 

 mouth, and whose cavity opens to 

 the exterior by paired proboscis 

 pores. The collar, which has paired 

 collar cavities with pores, is pro- 

 vided with from four to six plume- 

 like tentacles on the dorsal side. 

 These tentacles are provided with 

 cihated grooves which sweep food toward the mouth. There is but 

 one pair of pharyngeal clefts opening in the trunk just back of the 

 collar. The notochord is a slender diverticulum of the proboscis re- 

 gion of the dorsal wall of the ahmentary canal, practically identical 

 with that of Balanoglossus. The nervous system is not tubular but 

 is a mere plexus of nervous tissue on the dorsal epidermis of the collar. 

 The trunk is short and plump, and has paired body cavities in which 

 he the paired gonads. The ecology and habits of Cephalodiscus are 

 similar to those of other colonial sessile forms and especially like those 

 of some of the colonial ascidians. They hve in comparatively deep 

 water attached to the bottom, forming large colonies, each individual 

 of which is embedded in a hollow pocket of the common "house." 



Fig. 37. — Cephalodiscus dodeca- 

 lophus, anterior view. 1, tentacles; 

 ^, proboscis (buccal shield); 3, pig- 

 ment band on proboscis; 4, buds; 

 5, pedicle; 8, trunk. (From Hegner, 

 after Mcintosh.) 



