lOs 



stærke. bagiulrettede dorsale Toriie. De sammen- 

 satte Øine bar tiltaget i Stoi-relse, og deres Syns- 

 elemeiiter er lui noksaa tydelige. Umiddelbart neden- 

 for dem ligger det enkle Øie, som har bibeholdt sit 

 Ud?eende temmelig nforandret. Af de 2 Lappe, 

 hvori hos sidste Larvestadinm enhver af de blind- 

 sækformige Udvidninger af Tarmen var delt, viser 

 den dorsale allerede en Antydning til en yderligere 

 Klovning. Ungen svommer om i Våndet paa en 

 fra samme hos Larverne meget forskjellig Maade, 

 idet Bevægelsen nn ikke længere er stodvis, men 

 mere har Characteren af et ganske jevnt Lob, be- 

 virket ved hastigt paa hinanden folgende Slag af 

 Aarernes ydre Parti. Dyret vender herunder snart 

 Evg- snart Bngside opad og gjor ofte kredsformige 

 A'olter i Våndet. 



Paa Tab. XIV er afbildet fra venste Side 2 

 umiddelbart folgende Stadier (Fig. 4 og 5), hvo)-af 

 vil sees, at Skallen hnrtigt voxer i Størrelse, saa 

 at den tilsidst er tilstrækkelig rummelig til at Le- 

 gemet fuldstændigt kan trækkes ind i samme. Skal- 

 lens Form er imidlertid temmelig uforandret, skjøndt 

 maa.ske noget mere oval end i Lste Stadium, og der 

 er fremdeles ingen concentriske Linier at se paa 

 Valvlerne. Hovedet er nu ved en tydelig dorsal 

 Indbugtning sondret fra Xakkesegmentet, og den 

 umiddelbart foran Indbiigtningen liggende Del af 

 dets dorsale Flade begynder at hæve sig i Veiret 

 for at danne det for det voxne Dyr saa eharacteri- 

 stiske Fastheftningsorgan, hvilket dog endnu længe 

 udgaar fra Hovedet med en forholdsvis bred Basis. 

 Pandedelen begynder lidt efter lidt at antage den 

 for det voxne Dyr characteristiske koniske Form, 

 og under den sees allerede en temmelig dyb Ind- 

 bugtning, hvorved den sondi-es fra det nu triangu- 

 lært fremspringende Rostrum. Iste Par Folere be- 

 gynder at forlænge sig og antage en smalt kolle- 

 dannet Form, men er endn\i hos det sidste af de 

 her omhandlede 2 Stadier (Fig. 5) simple, uden late- 

 rale Lappe. Aarerne, hvis basale Del nu altid er 

 fortilstrakt, forandrer sig ligeledes gradvis, an- 

 tagende mere og mere den for det voxne Dyr cha- 

 raeteristiske Bygning, idet (Trenene forlænger sig 

 og afsnores i et storre Antal Led. Antallet af de 

 paa dem fæstede Svommeborster og Pigge er dog 

 endnu meget ringe sammenlignet med samme hos 

 det voxne Dyr. Kindbakkerne har tabt den sidste 

 Uest af Mandibularfodderne og er nu, saavel hvad 

 Form som Bevæbning angaar, fnldkommen overen.s- 

 stemmende med samme hos fuldt udviklede Individer. 

 Branehialfoddernes Tal tiltager gradvis, idet der 

 sueeessivt Ijagtil danner sig nye Anlæg, efterat de 

 foranliggende er hleven functionsmæssigt udviklede. 

 Deres Tal er paa det Fig. 4 af bi hiede Stadium 9, 



eaiidal laminæ. Above the caudal setæ moreover, 

 there have developed a single pair of the strong, 

 backward-pointing, dorsal .spines occurring here in 

 the full-grown animal. The compound eyes have 

 increased in size, and their visual elements are now 

 fairly distinct. Immediately below them lies the 

 ocellus, which has maintained its appearance almost 

 unchanged. Of the 2 lobes into which each of the 

 (•æcal expansions of the intestine was divided in 

 the last larval stage, the dorsal one already shows 

 signs of further cleavage. The yoxing animal swims 

 about in the water in a manner very different to 

 tliat of the larva, the motion being no longer jerky, 

 but having more the character of an even dart, 

 broxight about by the rapidly succeeding strokes of 

 the distal part of the oars, during which the ani- 

 mal turns now its dorsal, now its ventral side 

 uppermost, and often makes circular turns in the 

 water. 



On PI. XIV, 2 immediately following stages 

 (tigs. 4 and o) are illustrated from the left side. It 

 will be seen from these that the shell is rapidly 

 increasing in .size, until at hist it is sufficiently 

 capacious to allow of the body being completely 

 withdrawn into it. The form of the shell, however, 

 is almost unchanged, though perhaps somewhat 

 more oval than in the iirst stage, and there are 

 still no concentric lines to be seen on the valves. 

 The head is now divided from the cervical segment 

 by a distinct dorsal depression, and that part of 

 its dorsal surface immediately in front of the de- 

 pression, begins to be raised up in order to form 

 the organ of attachment so characteristic of the 

 adult animal, which still for some time issues from 

 the head with a comparatively In-oad base. The 

 frontal region begins little by little to assume the 

 conical form characteristic of the full-grown animal, 

 and beneatli it may be already seen a rather deej) 

 de])ression, liy which it is divided from the now 

 triangularly ])rojeeting rostrum. The 1st pair of 

 antennæ begin to lengthen and assume a narrow 

 clavate shape, but ai'e still, in the latter of the 

 2 .stages now in (luestion (tig. 5), simple and without 

 lateral lobes. The oars, whose basal part is now 

 always extended forwards, are also changing gra- 

 dually, and assuming more and more the structui-e 

 characteristic of the adult animal, the rami being- 

 elongated and divided into a greater number of 

 joints. The number of natatory bristles and spines 

 attached to them is still very small compared witli 

 that in the adult animal. The mandiljles have lost 

 the last remnants of mandibular legs, and are now, 

 both as to form and e(|ui|)ment, exactly like those 

 of the fully-developed animal. The numlier of 

 branchial legs increases gradually, new rudiments 

 lieing suct'cssively formed behind, as soon as those 

 in i'roiit ai'e develojicd sufliciently to perforin their 



