TUNICATA. 



[ 788 ] 



TUNICATA. 



The flattened epitlielial cells are also often 

 arranged around the papillae in the form of 

 concentric rings or nests, resembling fibres ; 

 but the cell-structure is at once rendered 

 e-^ident by the addition of solution of 

 potash. 



In regard to cells generally, an insuper- 

 able difficulty is met vdih in discovering 

 the exponent of their power, as it might be 

 temied ; thus the embryonic cells or cor- 

 puscles in an early stage are undistinguish- 

 able from each other, yet some grow into 

 fibres of connective tissue, others into 

 nerve-tubes, &c. Chemistry lends no aid 

 here, and the difficulty will probably ever 

 remain. 



In the examination of tumom's and other 

 morbid g:i'owths, sections should be made 

 with a Valentin's knife, the elements being- 

 first observed in water, and then in the 

 natm-al fiiud. The sections and elements 

 are best preserved in water. 



BiBL. Paget, Tumours and Surgiccd Por- 

 tJioloyy ; Bennett, on Cancer, and Ed. Mn. 

 Jn. vii. & viii. ; Redfern, ibid. xi. ; and the 

 Tr. Pathol. Soc, passim ; Green, Path. 

 Anatomy ; Rindfleisch, Path. Gewehel. 



TUNiCA'TA.— A class of Animals, be- 

 longing to the Invertebrate subkingdom Mol- 

 lusca. 



Char. Marine ; often microscopic ; bodies 

 single, social, or aggregate ; acephalous ; 

 enclosed in an elastic tunic with two orifices, 

 one oral and branchial, the other anal 

 or cloacal ; a large atrial system, the 

 pharynx commimicatiug with it by bran- 

 chial slits ; respiration branchial, branchife 

 phar}Tigeal ; nervous system a ganglion ; 

 circidation effected by a tubular heart, with 

 vessels, the current of blood varying in di- 

 rection ; hermaplu'odite ; evolution accom- 

 panied by metamorphosis, or following the 

 law of alternation of generations (PI. 18. 

 figs. 10 & 20). 



The smaller Tunicata are commonly found 

 aggregate, and investing rocks, stones, and 

 shells ; some are adherent to seaweeds, &c., 

 a few are free ; many are common on the 

 sea-shore. 



The body is sac-shaped or elongate, some- 

 times slightly constricted so as to exhibit a 

 thorax, abdomen, and a posterior portion or 

 postabdonien. The outer coat, test or 

 tunic, is cartilaginous, leathery, gelatinous, 

 or membranous ; and consists partly of cel- 

 lulose, often containing calcareous spicula. 

 Within this is another coat, the mantle, 

 usually adhering to the former at the orifices 



only, and containing numerous muscidar 

 fibres. The oral orifice (PI. 18. fig. 10, a), 

 or opening of the usually dilated pharynx 

 or branchial cavity, within wliich is placed 

 the branchial apparatus (6), leads to the 

 mouth. This is slit-like, and leads into a 

 narrow oesophagus; to this succeeds an 

 expanded stomach (/), which terminates 

 in a longish intestine with a haemal curve 

 (i), and ends at the base of the cloaca 

 {{*), which opens externally at the atrial 

 orifice. Within the oral orifice, at the com- 

 mencement of the branchial cavity, is a ring 

 of rudimentary tentacles. 



The branchial apparatus in the pharynx 

 (fig. 10, h) consists of numerous cross bars, 

 «ith slit-like openings between them ; these 

 are ciliated, and copiously supplied with 

 networks of blood-vessels. The slits open 

 into the atrial system. The current excited 

 by the cilia draws the water through the 

 oral orifice into the pharynx, where it tra- 

 verses the openings, flowing outwards to 

 collect in the cloaca, from which it is ex- 

 pelled through the atrial orifice. In some 

 of the larger Tunicata^ the branchial appa- 

 ratus is stra]^)-shaped, and traverses the bodj' 

 obliquely. 



Tlie heart {h) is a spindle-shaped sac, 

 enclosed in a pericardium (p), and situated 

 near the base of the body, the principal 

 vessels running on the dorsal and ventral 

 surface of the branchial aj^paratus. The 

 current of blood varies in direction, being at 

 one time expelled from one end of the 

 heart, at others from the other. In those 

 Tuuicata which are connected by a common 

 tube, the blood passes freely from one to 

 the other. 



The nervous system consists of a single 

 ganglion (fig. 10, g), situated between the 

 two orifices, and giving oft' its principal 

 branches to the branchial sac and the ali- 

 mentary canal. In some an eye is present, 

 resembling the compound eye of the Arti- 

 culata, and with a reddish pigment. 



Moreover an auditory capsule has been 

 noticed in some genera. 



The liver (/J consists either of a dark 

 glandular layer lining the alimentary canal, 

 or of distinct glandular ctieca. 



The Tunicata are reproduced by gemma- 

 tion, by sexual organs, and by intermediate 

 generations. 



The testis (/) and ovaiy {o) are usually 

 strap-shaped organs, either adherent to the 

 alimentary canal, or situ.ated in the poste- 

 rior part of the body j the former has a long 



