1218 



TUNICATA. 



to the mother by the former, and which at 

 first has a circulation in common with the 

 older animal, but soon enjoys an independent 

 existence. The individuals either remain at- 

 tached by the intermediate root-like prolonga- 

 tions, or become separate by the disunion of 

 these slender filaments. 



In one species of Clavellina, the bud-like 

 bodies and young animals are produced not 

 only on the root-like stolons, but also on the 

 walls of the test itself (fig. 768. u, u f ). 



In Perophora the growth of the buds is con- 

 fined to the extremity of the creeping, root- 

 like tube, and the attachment of the indi- 

 viduals by the common root-stem is perma- 

 nent through life, and not merely existent 

 during the young state. 



ANATOMY OF THE BOTRYLLID^. A de- 

 scription of the form and structure of the test 

 of these compound Ascidians has been already 

 given. The in- and out-lets of the aggregated 

 individuals, as we have already seen, have 

 either separate external openings, or the several 

 vents of a " system " of animals unite to form 

 one large common cloaca and external anal 

 orifice. 



All these external orifices are more or less 

 irritable and contractile. When the little 

 animals dilate their branchial orifice, a part of 

 their body is raised so as to slightly emboss 

 the general surface of the mass, and they pro- 

 trude a membranous circlet or ring, the free 

 border of which is cut into, generally six, regu- 

 lar lobes, but sometimes into eight. In Poly- 

 clinum, the mouth is very contractile, and is 

 surrounded by six little digitiform processes. 

 Parascidia has eight of these tentaculiform 

 bodies. Frequently, after death, as in Lepto- 

 clinum, these orifices contract, and their bor- 

 ders sink in so as to be discerned with diffi- 

 culty. In the interior of this orifice, and 

 towards the base of the collar, a series of 

 minute tentacular filaments can be perceived 

 by the aid of a lens, which are directed, like the 

 spokes of a wheel, towards the centre of the 

 opening, or have a more or less curled ap- 

 pearance, in a similar manner to the tentacles 

 occupying the same position in the simple 

 Ascidians and in the Clavellinidce. The ten- 

 tacles vary from four to twelve or more, but 

 nine or ten is their usual number ; they differ 

 materially in length, some being nearly ru- 

 dimentary, whilst others, alternating with the 

 first, are long enough to meet across the open- 

 ing. In Amaroucium Nordmanni there are six 

 long and six short tentacles within the bran- 

 chial tube. In Bolryllus smaragdus, in which 

 the buccal orifice admits of considerable dila- 

 tation, a circlet of four longish filiform ten- 

 tacles may be seen. The tentacles in B. 

 aureus are more numerous than usual, but 

 both in this species and in B. violaceus they 

 are almost rudimentary. Diazona has fifteen 

 or sixteen simple tentacles, and in Sigillina 

 there are twelve. 



Around the base of the denticulated rim of 

 the branchial orifice in Amaroucium argus 

 (Jig. 782.) there are seen four minute pinkish 

 spots, which are probably rudimentary organs of 



sight. Similar, but more numerous, pigment- 

 spots are observed in Leptoclinum Listen, Pa- 

 rascidia, and other genera. 



Fig. 782. 



Anatomy of Amaroucium argus. (After Milne* 

 Edwards.) An isolated individual considerably 

 magnified. 



A, thorax ; B, superior abdomen ; c, post-abdomen ; 

 a, proper tunic of the individual ; b, thoracic tunic ; 

 V, longitudinal muscular fibres of the inner tunic ; 

 c, branchial orifice ; d, branchial collar, below which 

 are seen the oculiform points; e, branchial sac; 

 /, thoracic sinus ; /', transverse vessels of the bran- 

 chial sac ; h, cloaca ; i, anal orifice ; ?', and appendage 

 or languet; jf, nerve-ganglion; k, oesophagus; Z, 

 stomach; m, intestine; n, anus, opening into the 

 cloaca ; o, heart ; o', pericardium ; p, ovary ; p', p", 

 ova, passing towards the cloaca ; g, testicle ; r, r>, 

 vas deferens, and its opening in the cloaca. 



The separate anal orifices are frequently 

 destitute of the crenulated or denticulated 

 margin that surrounds the branchial tube. 

 This condition has afforded a distinguishing 

 character for some of the minor groups in 

 Milne Edwards' classification of the Bo- 

 tryllidcs. In Polyclinum, Amaroucium, Bo- 

 tryllus, and other genera, the anus, instead of 

 opening directly outwards, empties itself into 

 a common cloacal cavity, that belongs to a 

 number of individuals, and is in the form of 

 a large canal hollowed out of the common 

 tegumentary mass. This is frequently rami- 

 fied interioVly, as in Botrylloides, and termi- 

 nates at the opposite extremity by a simple 



