OBSERVATIONS ON POLYZOA. 153 



We are, also, obliged to abandon any comparison be- 

 tween the muscular systems, which, although so compli- 

 cated and prominent in the polyzoon, is entirely wanting 

 in the Ascidian. 



No such radical differences occur in the Brachiopod. 

 It is only necessary to shift the positions of the various 

 organs and follow out to their consummation in the higher 

 group the structural changes indicated in the prototypical 

 Polyzoon. The arms, the lophophore, the tentacles, the 

 muscles, the simple dorsal flexure of the alimentary canal 

 are all present, and occupy about the same relative posi- 

 tions. 



By means of his diagrams Prof. Allman proves, that 

 the organs of the Plumatella, equivalent to those of 

 Clavellina, are respectively as follows : the ectocyst to the 

 external tunic, the endocyst to the internal tunic, the 

 calyx to the investing membrane of the branchial sac. 

 the tentacles to the transverse respiratory bars of the 

 branchial sac, the epistome to the " languettes." 



Besides the homologies already quoted from Hancock 

 and Huxley, I may add that the ectocyst, and endocyst 

 also, are found in the Brachiopod. 



If a section of a Terebratula be made along the autero- 

 posterior axis, and compared with a similar section of an 

 invaginated Polyzooid, the number of tunics are the same. 

 The shell is identical with the ectocyst, and has besides 

 the columnar structure of the columnar, calcareous ecto- 

 cyst of Eschara, which is similarly perforated by minute 

 follicles.* 



The mantle proper lining the shell is the same as the 

 endocyst, which carpets the interior of the ectocyst in 

 Polyzoon, and the inner side of the mantle corresponds to 

 the invaginated polypidal endocyst, or tentacular sheath, 



* Milne Edwards. Ann. des Sciences Nat., vol. 6, 183G. The As- 

 cidia, also, have similar processes, but these have afferent and efferent 

 canals, and, as Dr. Carpenter in the Proc. Eoyal Soc., vol. 7, p. 36, 

 1854, has pointed out, they do not resemble them closely in structure. 

 The coecal tubes of Terebratula, according to Hancock (Op. cit.), are 

 only prolongations of the outer membrane, whereas those of Eschara 

 open into the visceral cavity ; thus neither in Ascidia or in Brachiopoda 

 are they closely alike, although probably homologous organs in both. 



COMMUNICATIONS ESSEX INSTITUTE, VOL. V. 21 AUG., 1867. 



