^§ 89, 90. THE ECHINODERMATA. 91 



• CHAPTER VII. 



RESPIRATORY SYSTEM. 



§ 89. 



The respiration of the Echinoderms is performed in various ways. Tliese 

 are : 1. By exclusively respiratory branchiae. 2. By organs serving 

 at the same time other functions. 3. By means of water passing 

 through the openings of the skin into the cavity of the body, and aerating 

 the blood through the capillary vessels of the viscera. 



With the Asteroidea, Synaptinac, Sipunculidae and Echiuridae, every 

 individual has always two of these modes of respiration, and sometimes 

 all three, as with the Echinidae and Holothurinae. 



§ 90. 



I. Organs which are exclusively respiratory are found in the Echi- 

 nidae, Holothurinae, and Echiuridae. They consist of external branchiae 

 in the first,' and internal in the last two. 



The external branchiae of the Echinidae are situated upon the soft 

 membrane of the mouth, being formed of five pairs of arborescent, hollow 

 lobules.'^* They are contractile, but cannot be retracted within the body. 

 They are covered both internally and externally with ciliated epithelium. 



The cavity of each communicates with that of the body by a large ori- 

 fice situated on the internal surface of the oral membrane. <-'' By this 

 means they are bathed with water upon both of their surfaces. Their 

 walls contain a coarsely reticulated calcareous skeleton, ^^' and without 

 doubt, also a capillary net-work belonging to the branchial vessels. 



The internal branchiae of the Holothurinae arise as two tubes from the 

 cloaca of the intestinal canal, and send off, through the whole cavity of 

 the body, numerous coecal branches.*^' In Holothuria tubulosa, one of 

 these tubes is closely connected with the turns of the intestine, while the 

 other is attached to the inner walls of the body. With the first, espe- 

 cially, may be perceived the ramifications of the branchial vessels. They 

 are also covered with ciliated epithelium, and their contractile and expan- 



1 The ramified organs of tlie Echinidae, already been very well described by Tiedemann (loc. cit. 

 known by Tiedemann (loc. cit. p 78, Taf. X. fig. p. 11, Taf. 11. or Wagner Icon. zoot. Tab. 

 5, d. d.) and De/le Chiaje (loc. cit. II. p. 338), XXXII. fig. 9), and by Delia C hiaj e (\oc. cit. 

 have been more exactly described by Valentin Tav. VIII. IX.). See also Atlas Zool. du Voyage 

 (Monogr. &c. p. 82, PI. IV. fig. 57 ; PI. VHI. fig. de 1' Astrolabe. Zoophytes, PI. VII. fig. 2, 9, p. 

 42), and by Erdl {JViegmanrCs Arch. 1842, I. (Holothuria ananas) and PI. VII. fig. 3, e (Clad- 

 p. 59, Taf. II. fig. 12, 13). olabes spinosus). Pentacta doliolum has simi- 



2 Valentin, loc. cit. PI. VII. fig. 135, 1. lar organs. According to Cuvier (Anat. Comp. 



3 Valentin, loc. cit. fig. 143 ; and Erdl, loc. cit. VII. 1840, p. 536) there is only a single branchia 

 fig. 13.* in the other remaimng Holothurinae. 



< The branchiae of Holothuria tubulosa have 



* [§ 90, note 3.1 See, in this connection. Mailer tin^s observations as to the structvire of the exter- 

 (Arch. 1850, p. 122), who has confirmed Fa^en- nal gills. — Ed. 



