Fig. 226. Second stage of Erichthus larva of Squilla with five maxillipeds and the first pair of abdominal appendages. (From Claus.)
The youngest known Erichthus form is about two millimetres in length, and has the characters of a modified Zoæa ([fig. 226]). The body is divided into three regions, an anterior unsegmented region to which are attached two pairs of antennæ, mandibles, and maxillæ (two pairs). This portion has a dorsal shield covering the next or middle region, which consists of five segments each with a pair of biramous appendages. These appendages represent the five maxillipeds of the adult[192]. The portion of the body behind this is without appendages. It consists of three short anterior segments,—the three posterior thoracic segments of the adult,—and a long unsegmented tail. The three footless thoracic segments are covered by the dorsal shield. Both pairs of antennæ are uniramous and comparatively short. The mandibles, like those of Phyllopods, are without palps, and the two following pairs of maxillæ are small. The five maxillipeds have the characters of normal biramous Zoæa feet. From the front of the head spring a pair of compound eyes with short stalks, which grow longer in the succeeding stages; between them is a median eye. The dorsal shield is attached just behind this eye, and is provided, as in the typical Zoæa, with a frontal spike—while its hinder border is produced into two lateral spikes and one median. In a larva of about three millimetres a pair of biramous appendages arises behind the three footless thoracic segments. It is the anterior pair of abdominal feet ([fig. 226]). The inner ramus of the second pair of maxillipeds soon grows greatly in length, indicating its subsequent larger size and prehensile form ([fig. 227] g). When the larva after one or two moults attains a length of six millimetres ([fig. 227]) the abdomen has six segments (the sixth hardly differentiated), each with a pair of appendages (the two hindermost still rudimentary) which have become gradually developed from before backwards. The three hindermost thoracic segments are still without appendages.
Fig. 227. Advanced Erichthus larva of Squilla with five pairs of abdominal appendages. (From Claus.)
f. first maxilliped; g. second maxilliped.
Some changes of importance have occurred in the other parts. Both antennæ have acquired a second flagellum, but the mandible is still without a palp. The first and second pair of maxillipeds have both undergone important modifications. Their outer ramus (exopodite) has been thrown off, and a gill plate (epipodite) has appeared as an outgrowth from their basal joint. Each of them is composed of six joints. The three following biramous appendages have retained their earlier characters but have become much reduced in size. In the subsequent moults the most remarkable new features concern the three posterior maxillipeds, which undergo atrophy, and are either completely lost or reduced to mere unjointed sacks ([fig. 228]). In the stage where the complete Erichthus type has been reached, these three appendages have again sprouted forth in their permanent form and each of them is provided with a gill sack on its coxal joint. Behind them the three ambulatory appendages of the thorax have also appeared, first as simple buds, which subsequently however become biramous. On their development the full number of adult appendages is acquired.
