14. Later stage (protozoea). Thf carapace has increased iu size. Compound eyes and frontal sense organs ( f) have 



appeared, and the appendages (IV -VII) which before were functioiiless rudiments have developed iuto 

 bJramous swimming organs. The mandible has become reduced to a cutting blade without palpus. The 

 hinder portion of the hody has greatly increased in size, and behind the seventli pair of appendages is the 

 indication of a number of somites. 



15. Mouth parts of the same, seen from below. The labrum {I b) is produced into a prominent spine. 



16. Paired eyes of a little older larva. 



Note. — Although the stages represented in the preceding figures were captured free-swimming' on the 



surface of the sea, and no connection directly established between the several stages or with the parent, 



there is no reason to doubt the correctness of MiiUer's identification of the forms as young stages of Pcncus. 



17-21. From Claus, Untersuchungen zur Erforschung der Genealogischen Grundlage des Crustaeeen-Systems, 



Taf. II., UI., Wien, 1876. 



17. Piotiizoea stage oS Pcneiis. IJmm. long, ocl, ocellus, oc, paired eyes under the carapace, as in adult of 



gvmis A) phcus. /, frontal sense-organ, similar to that of Phyllopoda. Behind the second maxillipeds (VII) 

 are six free thoracic segments. Under the cuticle of the first of these are seen the rudiments of the third 

 maxillipeds (VIII). The abdomen (ab) has no free segments yet, but under the skin can he seen the 

 segmentation which causes the six abdominal somites of the next stage. 



18. Si.x thoracic somites and abdomen of a somewhat larger laiTa, zoea form. The five anterior somites are now 



free, the sixth is not yet separated from the telson. It appears that the somites of the thorax and abdomen 

 develop in regular succession from before backwards. The third maxilliped (VIII) is now a free bilnbed 

 bud, and behind it are seen the rudinnnts of the following five pairs of tlioracic feet. In the abdominal 

 section of the body the last pair of limbs is apparent as a small bilobed process (XIX) on the sixth somite, 

 and perhaps the slightest trace of the other five pairs is already perceptible. 



19. Older zoea form of the same. The paired eyes are now freed from the carapace and mounted on long stalks. 



The five posterior pairs of thoracic appendages (those of the left side have been removed iu the figure) have 

 developed into jiroininent biramous sacs, while the third maxillipeds (^III) are furnished with setie on 

 each branch. The abdomen has become very long, the telson is separated from the sixth somite by a mov- 

 able joint, and the posterior appendages (XIX) have assumed the shape of powerful swinimerets. The second 

 antenna; still serve as locomotive organs, in which office the muscular abdomen now assists. The five 

 anterior abdominal somites have not yet developed appendages, at least to any functional degree. The 

 development of the sixth abdominal appendages thus anticipates the development of the anterior pairs, 

 probably on account of their functional importance as swimming organs. 



20. Schizopod or Mysis stage of a Peiieus. 16 mm long. As in the adult Mysis, the biramous thoracic feet now serve 



as swimming organs. All the abdominal limbs are present. The first antennie have lost their long set;e, 

 and grown an inner hranch which becomes the inner flagellum of the adult appendage. The second an- 

 tennje have resigned their locomotive office, and their outer branch has become reduced to the antennal 

 "scale" (II')- The uauplius eye has disappeared, and mandibular palpi have developed. The transition 

 from the Mysis stage to the adult is easy, the most marked change consisting in the reduction of the exter- 

 nal branches of the five pairs of ambulatory appendages to rudimeutary structures. 



21. Telson of the same stage. 



