384 



BoTUVLUD.E.- 



-MOLLUSCA.- 



-D1ST0511S. 



jections. These are Compound Ascidians. A tangle 

 or broad-Icavcd fuciis, torn from its rocky bed, or 

 gatlicrcd on the sands whore the waves have cast it 

 lifter storms, will show us similar bodies, mostly those 

 star-figured, investing its stalks, winding among the 

 intricacies of its roots, or clothing with a glairy coat the 

 expanse of its foliated extremities. If we keep some of 

 these bodies in a vessel of sea-water we find they lie 

 ns apathetic as s]Kinges, giving few symptoms of vitality 

 beyond the slight ponling out of tube-like membranes 

 around apertui'cs which become visible on their sur- 

 faces, though a closer and microscopic examination will 

 bIiow us currents in active motion in the water around 

 these apertures, streams rejected and whirlpools rush- 

 ing in; indicating that however torpid the creature may 

 externally appear, all the machinery of life, the respira- 

 tory wheels and circidatory pumps, are haid at work in 

 its iinnost recesses. In the course of our examination, 

 especially if we cut up the mass, we find that it is not 

 a single ainnial whirb lies before us, but a common- 

 wealtii of beings bound together by common and social 

 tics. . . . I'iach member of the commonwealth has 

 its own peculiar duties, but shares also in operations 

 which relate to the interest and well-being of the mass. 

 Anatomical investigation shows us the details of these 

 curious structures and arrangements beautiful as wise. 

 Indeed few bodies among the lower forms of animal 

 life exhibit such exquisite and kaleidoscopic figures as 

 those which we see displayed in the combinations of the 

 Compound Ascidians." — {Forbes.) The Compound 

 Ascidians form rather a large family, comprising within 

 it at least thirteen distinct genera. These m.ay bo 

 arranged, however, in three groups or sub-families. 



Slii-iamii.y I.— BOTRYLLINA. 



In the animals of this group the individuals are 

 united in systems around connnon excretory cavities 

 or cloaca; ; and tlieir bodies are not divided into a dis- 

 tinct tlunax and abdomen. 



GKNt:s BoTliYM.us. — Tins genus may be taken as 

 the type of this group. The external tunic is gelatinous 

 or cartilaginous, sessile, and incrusting. The animals 

 are grouped in simple prominent stars, and the indi- 

 viduals — from six to twenty in each system — lie hori- 

 zontally, with the vent far from tlie branchial orifice, 

 which is simple. The braneliial orifices are arranged 

 round a connnon doaco. "The tests of tlie animals 

 are fixed together, forming a common mass, in which 

 the animals arc imbedded in one or more groups or 

 systems, but the individuals are not connected b}' any 

 internal union." — (Athims.) There have been as 

 many as ten distinct species described, six of winch are 

 found on the shores of Great Britain. They vary con- 

 siderably in form and colour, some of thera being 

 purple, and others yellow, blue, grey, or green. They 

 are fixed on stones or sea-weed, near low-water mark. 

 The genus is represented by B. pohjcijdus (Plate 9, 

 figs. 7 and 8). 



Sii!-FAMii.Y 11. -POLYCLININA. 

 This group is distinguished from the preceding by 



the body being divided into three distinct portions, viz. 

 — a thorax, with the branchial apparatus; a superior 

 abdomen, with the digestive organs; and a post- 

 abdomen, with the heart and reproductive organs. 



Gi:nus Polyclynum. — This genus has a gelatinous 

 or cartilaginous test, sessile or slightly pedunculated, 

 and varying very much in form. The systems are 

 numerous, convex, somewhat star-shaped, each with a 

 central cloacal cavity. The individuals, varying from 

 ten to an hundred and fifty, are placed at unequal 

 distances from their eonmion centre. The abdomen is 

 much smaller than the thorax, and the post-abdomen is 

 pedunculate. The branchial orifices are six-angled and 

 six-rayed, and the anal aperture is prolonged hori- 

 zontally and irregularly cut. Six species are known, 

 one of wdiich, P. conslellatwn, is figured in Plate 9, 

 figs. 12-14. 



Genus Synoici^m. — This genus has the test semi- 

 cartilaginous, cylindrical, and pedunculate, and the 

 animals are either isolated or attached gregariously to 

 similar cylindrical bodies. The systems are single, 

 circular, terminal, and comprise six to nine animals. 

 The branchial orifices are six rayed, and the anal 

 aperture has the same number; but they are unequal, 

 the three lai'gest forming the exterior margin of the 

 central star. Only one species is known, S. tiirgcns, 

 figured in Plate 9, figs. 15-17. 



Gi;nus Sigii.lina. — This genus has a solid, gelatin- 

 ous test, in form of an elongated, erect cone. It is 

 pedunculated, and occurs either isolated or gregarious. 

 Each cone consists of a single system of many indivi- 

 duals disposed one above another in irregular circles. 

 The branchial and anal orifices have each six rays ; 

 the abdomen is larger than the thorax ; and the post- 

 abdomen is long and slender. The genus derives its 

 name from a fancied resemblance of the common mass 

 to a seal (sigilhim), and contains as yet only one species, 

 S. oustro/is, from the tropical seas, which is repre- 

 senlod in Plate 9, figs. 10 and 11 {magnified). 



SuD-i-.\Mii.Y III.-DIDEMNINiE. 



This group embraces all the compound Ascidians 

 whose body is distinctly divided into two parts, tlia 

 thorax and abdomen. 



Genus Didkmnium. — This genus has a coriaceous 

 test which is sessile and encrusting, and diversified in 

 form and appearanc(^ The systems are numerous, 

 compressed, without central cavities, and the individuals 

 are scattered over the common body without any appre- 

 ciable order of arrangement. The abdomen is pedun- 

 culate. The branchial orifice only is rayed. 



Genus Distomu.-j. — This genus ditfers from the 

 preceding in having both the branchial and anal orifices 

 rayed. The test is semicartilaginous, sessile, and 

 assumes various forms. The systems are numerous, 

 usually circular, and the individuals are placed in one 

 or two rows at unequal distances from a common cen- 

 tre. The rays at the margin of both apertures are 

 six in number. There are several species of this 

 genus, one of wbicli, D. ruber, is represented in Plate 

 9, figs. 1-3, 



