104 



THE, YOUNG NATURALIST. 



ceedingly curious to note, the circulation 

 can be periodically reversed, so that alter- 

 nately the two ends of the heart are arterial 

 and venous, the blood being forced a definite 

 number of times in one direction and then 

 the action being reversed for as many times 

 in the opposite direction. With the excep- 

 tion of the aberrant form, A^pendicularicL — 

 which, however, may be as Mr. Saville Kent 

 aptly remarks, " the free reproductive zooid 

 of an Ascidian, bearing the same relation to 

 the adult as the Medusas do to the Hydroid 

 colony," — the nervous system is exceedingly 

 simple indeed, consisting only of a single 

 ganglion placed between the branchial and 

 atrial apertures. 



Worthy of remark is the developement of 

 a sea-squirt. The embryo is generally free- 

 swimming, resembling much in outward 

 form our common tadpole. In process of 

 growth a peculiar structure, very closely 

 similar to the chorda dorsalis of Vertebrates, 

 appears; and the nervous system, according 

 to Mr. Kowalewsky, is developed in a dorsal 

 cavity quite distinct from the one in which 

 the viscera are contained (Mem. Acad. Imp. 

 St. Petersburg, Ser. vii., Tom. x., 3., 1866). 

 This, however, seeing that if snch be the 

 case, they bear an exceedingly close resem- 

 blance to the embryo structure of the Verte- 

 brata, has been disputed by several eminent 

 naturalists, among them Von Baer, Reichert 

 and Donitz (See " Entwickelt sich die larve 

 der einfachen Ascidien in der ersten Zeit 

 nach dem Typus der Wirbelthiere." — Von 

 Baer, M^m de I'Acad. Imp, des Sciences de 

 St. Petersburg, 1873 ; and Zur Anatomie 

 des Schivanzes der Ascidien-Larven (Bo- j 

 tryllus violaceus), H. Reichert, Abhandl. 

 d. k. Akad. der Wiss., Berlin, 1875). It is 

 useless, however, for me to discuss this 

 relationship, and I think I cannot do better 

 than quote the excellent summary of the 

 researches of these natmralists as given by 

 Professor Nicholson : — 



I. The axial rod of the larval Ascidian 

 is ventral in position and cannot therefore 



be homologically compared with the chorda 

 dorsalis of the Vertebrate embryo. (Von 

 Baer). 



2. The embryo of the Tunicates is not 

 really bi-cavitary, and the nervous system 

 is not developed in a chamber separate 

 from, and lying above, the visceral cavity. 

 (Donitz, Reichert). 



3. The nerve-ganglion of the Tunicates 

 is placed upon the ventral surface of the 

 larva and does not, therefore, correspond 

 with the cerebro-spinal nervous system of 

 the Vertibrates. (Von Baer). 



^. The tail of the larval Ascidian is a 

 purely provisional organ. 



5. The axial structures of the tail con- 

 sist of a central, homogeneous, structureless, 

 and elastic rod, surrounded by a sheath of 

 large nucleated cells, and the former is not 

 primitively composed of cells. (Reichert). 



6. The perforated branchial sac is not a 

 dilated pharynx, but appears to correspond 

 rather with the branchial chamber of the 

 Lamellibranchiata." (Manual of Zoology, 

 p. 368;. 



From the above summary you will see, 

 taking into consideration the researches of 

 Kowalewsky, that zoologists are not as yet 

 perfectly agreed as to whether this impor- 

 tant phenomena manifested in the develop- 

 ment of a sea-squirt has or has not an 

 homology with the chorda dorsalis or note- 

 chord of the Vertebrata. 

 Mitton, Stourport, March, 1883. 



ENTOMOLOGICAL 

 LOCALITIES, 



No. 4. 



DEAL TO ST. MARGARET'S BAY. 

 By W. H. TuGWELL. 



On page 80, of the Y.N., I gave a short 

 account of the Deal Sandhills, lying to the 

 north of Deal ; here it is purposed to sketch 



