108 



stærlie, hagudrettede dorsale Torne. De sammen- 

 satte Øine har tiltaget i Størrelse, og deres Syns- 

 elementer er nu noksaa tydelige. Umiddelbart neden- 

 for dem ligger det enkle Øie, som har bibelioldt sit 

 Udseende temmelig nforaiidret. Af de 2 Lappe, 

 hvori hos sidste Larvestadium enhver af de blind- 

 sækformige Udvidninger af Tarmen var delt, viser 

 den dorsale allerede en Antj'dning til en yderligere 

 Klovning. Ungen svømmer om i Våndet paa en 

 fra samme hos Larverne meget forskjellig Maade, 

 idet Bevægelsen mi ikke længere er stedvis, 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 

 Ryg- snart Bugside opad og gjor ofte kredsformige 

 Volter i Våndet. 



Paa Tab. XIV er afbildet fra venste Side 2 

 umiddelbart folgende Stadier (Fig. 4 og ,o), hvo)'af 

 vil sees, at Skallen hurtigt 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 

 maaske noget mere oval end i Iste Stadium, og der 

 er fremdeles ingen concentriske Linier at se paa 

 Valvlerne. Hovedet er nu ved en tydelig dorsal 

 Indbugtning sondret fra Nakkesegmentet, og den 

 umiddelbart foran Indbugtningen 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 eharacteristiske koniske Form, 

 og under den sees allerede en temmelig dyb Ind- 

 bugtning, hvorved den sondres fi-a det nu triangu- 

 lært fremspringende Rostrum. Iste Par Følere be- 

 gynder at forlænge sig og antage en smalt kolle- 

 dannet Form, men er endnu 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 eha- 

 raeteristiske Bygning, idet Grenene forlænger sig 

 og afsnores i et større Antal Led. Antallet af de 

 paa dem fæstede Svommeborstor og Pigge er dog 

 endnu meget ringe sammenlignet med samme lujs 

 det voxne Dyr. Kindbakkerne har tabt den sidste 

 Rest af Mandibulai'fodderne og er nu, saavel hvad 

 Form som Bevæbning angaar, fuldkommen overens- 

 stemmende med samme hos fuldt udviklede Individer. 

 Branehialfoddernes Tal tiltager gradvis, idet der 

 successivt bagtil danner sig nj^e Anlæg, etterat de 

 foranliggende er bleven functionsmæssigt udviklede. 

 Deres Tal er paa det Fig. 4 aflu'ldede Stadium 9, 



raudal 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 com])ound 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 

 cæcal expansions of the intestine was divided in 

 the last larval stage, the dorsal one ali'eady shows 

 signs of further cleavage. The young animal swims 

 aboi;t in the water in a manner very different to 

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

 but having more the character of an even dart, 

 brought aboTit 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 

 uiipermost, and often makes circular turns in the 

 water. 



On PI. XIV, 2 immediately following stages 

 (bgs. 4 and b) are illustrated from the left side. It 

 will be seen from these that the shell is rapidly 

 increasing in size, until at last it is sufficiently 

 capaeioi^s to allow of the body being completely 

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

 is almost unchanged, though perluips somewhat 

 more oval than in the first 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 broad l)ase. The 

 frontal region begins little by little to assume the 

 conical form characteristic of the full-grown animal, 

 and beneath it may be already seen a rather <lecp 

 depression, by which it is divided from the now 

 triangularly projecting rostrum. The 1st pair of 

 antennæ begin to lengthen and assiime a narrow 

 clavate .shape, but are still, in the latter of the 

 2 stages now in (|uestion (fig. o), simple and without 

 lateral lobes. The oars, whose basal part is now 

 always exteiuled forwards, are also changing gra- 

 dually, and assuming more and more the structure 

 characteristic of the adult animal, the rami being 

 elongated and divided into a greater number of 

 joints. The niimber of iiatatory bristles and spines 

 attached to them is still very small conijiared with 

 that in the adult animal. The mandibles have lost 

 the last remnants of mandibular legs, and are now, 

 both as to form and equipment, exactly like those 

 of the fully-developed animal. The number of 

 branchial legs increases gradually, new rudiments 

 being successively formed behind, as soon as those 

 in front are develo])cd sufricicntly to perform their 



