382 MOLLUSCA. 



projection on one part of the cone, which really resembles the two valves of the Acephales. The re- 

 semblance between its little tubes, and those which envelope the tentacula of certain Terebella, formerly 

 caused this animal to be referred to the Annelides. 



The best known species (Asp. jafanus) is seven or eight inches in length. [Rang conjectures that the animal 

 of Aspergillum is essentially the same as that of Clavagella, and, as well as Blainville, he erroneously thinks that 

 both are furnished with a byssus passing through all the anterior apertures of the tube, to attach it to foreign bodies. 

 Th.e Aspergillum probably burrows in sand, the disk underneath, and the tubular part uppermost.] 



THE SECOND ORDER OF THE ACEPHALES. 



THE SHELL-LESS ACEPHALES, (or A. nuda). * 



This is a small order, and differs so far from the otlier Acephales that it might be made a 

 distinct class, were such a division considered to be convenient. Their branchiae assume 

 various forms, but are never divided into four leaflets : the shell is replaced by a cartilaginous 

 tunic, sometimes so thin that it is as flexible as a membrane. We divide the order into two 

 families. 



THE FIRST FAMILY OF THE ACEPHALA NUDA,— 

 The Segregata, — 

 Embraces the genera whose individuals are isolated and without mutual organic connection, although 

 they often live in societies. 



The Biphores, Brug. (Thalia, Crown ; Saljja and Dagysa, Gm.), — 

 Have the cloak and its cartilaginous envelope oval or cylindrical, and open at the two extremities. On 

 the side of the anus the aperture is transverse, wide, and furnished with a valve, which allows the water 

 to enter, but prevents its egress; on the side of the mouth the aperture is simply tubular. Muscular 

 bands embrace the cloak and contract the body. The animal moves by forcing out from the anteiior 

 aperture the water which has entered the body by the posterior, so that its motion is always retrograde, 

 whence it has happened that some naturalists have mistaken the posterior aperture for the real mouth. 

 It also generally swims with the back undermost. The branchial form a single tube or riband, furnished 

 with regular vessels, placed obliquely in the middle of the tubular cavity of the cloak in such a manner 

 as to be constantly bathed by the water as it traverses that cavity.f The heart, the viscera, and the 

 liver, are piled near the mouth towards the back ; but the position of the ovary is variable. The cloak 

 and its envelope exhibit in the sun the colours of the rainbow, and are so transparent that the whole 

 structure of the animal can be seen through them : in many they are furnished with perforated tubercles. 

 The animal has been seen to come out from its envelope without apparently any injury. But a more 

 curious fact in their history is that, during a certain period, they remain united together, as they were 

 in the ovary, and float in the sea in long chains, the individuals being disposed, however, in a pattern 

 different in different species. M. de Chamisso assures us that he has ascertained a still more singular 

 fact, which is, that the individuals that have issued from a multiplicate ovary have not an ovary of the 

 same kind, but produce only isolated individuals of a form considerably different from their originals ; 

 and these again, give birth to others with ovaries similar to the parents of the first, so that there is, 

 alternately, a scanty generation of separated individuals, and a numerous generation of aggregated indi- 

 viduals, and these two alternating generations do not resemble each other. Certainly we have observed, 

 in some species, small individuals adherent to the interior of larger ones by a peculiar sucker, which 

 were difTterent in shape tiom those which contained them. These animals are found in abundance in 

 the Mediterranean and the warmer portions of the ocean, and are frequently phosphorescent. 



The Thalia, Brown, have a little crest or vertical fin near the posterior end of the back. 



Amongst the Salpa, properly so called, there are some which have, within the cloak, above the visceral mass, a 

 gelatinous plate of a deep colour, which may be tlie rudiment of a shell. In others there is only a simple protu- 

 berance of the cloak itself in this situation, but of a thicker texture. In others there is neither plate nur pro- 



• The .-/(r;)/ia/o/)/ii/i-a/ic/i;roir«ni7it(i(« of Blainville. The Titmailn I + Some authors say th:it this lube is pcrfiiratei! at both ciiils. aiul lliat 

 of Lainai-crk. | the water traverses it, a faet 1 have in vain sought to ileteruiiiie. 



