SS2 MOLLUSCA. 



projpcrion on one part of tlu' cone, wliicii really rcscmljles the two valves of tlic Acejiliales. The re- 

 semblance l)ft\verii its htthj ttihes, and those which envelope the tcntacula of ccrtuin Terebella, formerly 

 cansed this animal to be referred to the Annelides. 



The best known species [Asp. javanus) is seven or ei^ht inches In lenfrth. [Ran^'' coiijectiires tliat tlie animal 

 of Asper^illum is essentially tlie same as tliat of Ciava<fella, and, as well as lilainville, lie erroneously tliinks timt 

 both arefurnislicd with a byssus passin;^ through all the anterior apertures of the tube, to attach it to forei;;n bodies. 

 The Asperg-illam prubably burrows in sand, the disk underneath, and the tidjular parr 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 Ace|djales that it might be made a 

 (Uslinct cla^s, were such a division considered to be convenient. Their biauchitc assume 

 various forms, Init are never divideil into fuur leaflets : the shell is reidaccd l)v a cartilagnioiis 

 tunic, sometimes so thin tliat it is as flexible as a membrane. A\ e divide the order into two 

 families. 



THE FIRST FAMILY OF THE ACEPIIALA NUDA,— 

 The Segregata, — 

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

 thev often live in societies. 



The Biphores, Brug. {Thalia, Brown ; Salpa and Bagym, 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, wdhch allows the water 

 to cuter, hut prevents its egress; on the side of the mouth the aperture is simply lubular. Muscular 

 band; embrace the cloak and contract the body. Tlie animal moves by forcing out from the anteiior 

 aperture the water which has entered the Ix.idy Ijy the posterior, so that its mnliiui is always retrograde, 

 whence it has hap[)ened that some naturalists have mistaken the posterior aperture for the real mouth. 

 It also generally swims with the back undermost. The liranchiic form a i,ingle tube or riband, furnished 

 with regvdar 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 uioiilh towards the back ; but the position of the ovary is variable. The cloak 

 and its envelope exliilut in tin; mui the colours of the rainbow, and are so transparent that the whole 

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

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

 curious fact, in ilicir hi^torv is that, during a certain period, they remain united together, as ihey "were 

 in the i)\ary, and flijat in the sea in long chains, the individuals being disposed, however, in a pattern 

 difl'crcnt in ditlerent species. M. de Chamisso assures us that he has ascertained a still more singular 

 fact, which is, that the indiviiliials that have issued from a multi]dicate ovary have not an ovary of the 

 same kind, Imt produce onlv ii^olatcd individuals of a form considerably diiferent from their originals; 

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

 alternatelv, 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 uulividuals adherent to the interior of larger ones by a peculiar sucker, which 

 were different in shape tioni those wdiich contained them. These animals are found in abundance in 

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



The Thalia, Bniwn, liave a little crest or vertical tin near the posterior end of the back. 



Anianp;st the Salpa-, properly so called, there are some ■v>liich have, within the cloak, above the visceral mass, a 

 gelatinous plate nf a deep colour, which may be the rudiment of a slicll. In others there is only a Mui[ile i)rntu- 

 berance of the cloak itself in this situation, but of a thicker texture. In others there is neither plate nor pro- 



•• TIic .-Irrphnloph-jTu hclcrobniucliuiln oi \i\!s.u\\\\\c. Tliu T"'iic':ta I + Snnic :iutli.^rs sny Oi.it ttiK lube is pi.Tf.^raleri at hoiticiuls -tikI that 

 of Uii;i,-Lk. I tlic wiUcr tnucrs:;^ it, ^ fact 1 lia^-« hi v;,in soui,'lil to ikl.Tniiiic, 



