KONGL. sv. VET. AKADEMIENS HANDLINGAR. BAND 34. N:0 8. 



The position of tlic hypostomic eyes un 1 1 h • central surface of the trilobites is nol 

 quite so abnottnous, tior so isolated ;i feature ainongsl the crustaceans as migbl al firsl 

 be supposed. Aniongst the recenl crustacea there is probabl) iione, which a- to the cori 

 formation of its labrum <>r hypostoma reserablea the trilobites so closely as the species of 

 the genus Apus, however different they raay else !><• in all other respects. We have 

 studied chiefly the hypostoma of Apus cancriformis. In its shape as seeri from the out- 

 side and as to its outlines it reminds stroiiLily <>l the trilobite hypostoma. It i- nearly 

 square (pl. VI figs. 46, 47) with rounded corncrs. The anterior margin i- arched and in 

 the middle it has ;i broad tonguelike prolongation. On the sides of the .interiör margin 

 there are two short, sharply pointed wings. The lateral margins are thick, almosl as 

 doubled, and bent forward so that a groove is formed alongside them and the lar 

 evenly vaulted median surface of the hypostoma. These grooves are continued in a shorl 

 groove parallel with the posterier margin. From this margin a narrow oblique surface 

 is slanting backwards, as it were, doubling that margin. On its surface there are two 

 small, resplendent, white or clear spöts, almost of the shape of a crescent, though niore 

 irregular when seen in higher enlargement (pl. VI tig. 49). They are translucent as to 

 be visible on the interiör side of the hypostoma. In Apus glacialis there are also similar 

 spöts in the same position, but they form short narrow, straight stripes, parallel with the 

 börder of the posterior margin. I have no opinion as to the nature or function of these 

 white spöts. I do not think that they at all are homologous with the maculae of the 

 trilobites. On the other hand we see a little higher up on the exteriör side of the hypo- 

 stoma two faintly elevated elliptic tubercles which as to their position and shape highly 

 remind of the maculaa of the trilobites. They are hollow on the inside and we have not 

 been able to find any peculiar structure in them. That they have any visual function is 

 precluded through the circumstance that the whole exteriör surface is densely covered 

 with microscopic bristles (pl. VI hg. 49) which are especially remarkable along the pos- 

 terior margin. 



In the class of the Cirrhipedia a sufficient number of facts is known to ascertain 

 us of the presence of eye spöts on the hypostomas of the larva. The researches of 

 Spence Bate l show in the larva of Balanus balanoides, on the hypostoma or labrum, 

 which he calls proboscis, an eye, a little from the posterior margin of the labrum. It 

 seems during the various moults of the animal to have changed its place, being in the 

 two first moults situated near the anterior margin of the hypostoma. In Clitia Strömi it 

 lies at the anterior part of the labrum (figs. 9, 10). Spence Bate does not accept the 

 opinion of their nature as eyes, but the following authors as Darwin and Claparede 

 agree in considering these spöts as eyes. Darwin 2 describes two such eyes in Lepas 

 australis and, if I catch his meaning rightly, one on each side of the labrum. Claparede 

 has given a figure of the larva of Lepas anatifera 3 in the hypostoma of which (by him 



1 On the developmeiit of the Cirripedia, Ann. Mag. Nat. Hist. 1851, p. 324, pl. VI figs. 1, 2, 3, pl. 

 VII fig. 10. 



2 Monogr. of Cirripedia I p. 17. 



3 Beobachtungen . . . iiber wirbellose Thiere, 1863, pl. XVII fig. 22. 



